 
Time Routines in CSPICE
===========================================================================
 
   Last revised on 2015 SEP 09 by N. J. Bachman
 
 
Abstract
--------------------------------------------------------
 
   Time conversion routines included in SPICE provide conversions between
   several time systems -- Coordinated Universal Time (UTC), Barycentric
   Dynamical Time (TDB) and Terrestrial Dynamical Time (TDT) -- and most
   common time formats -- calendar, day of year, and Julian Date.
 
 
References
--------------------------------------------------------
 
   The formulation and the values used in this document are taken from the
   following sources
 
       1.   Moyer, T.D., Transformation from Proper Time on Earth to
            Coordinate Time in Solar System Barycentric Space-Time Frame of
            Reference, Parts 1 and 2, Celestial Mechanics 23 (1981), 33-56
            and 57-68.
 
       2.   Moyer, T.D., Effects of Conversion to the J2000 Astronomical
            Reference System on Algorithms for Computing Time Differences
            and Clock Rates, JPL IOM 314.5--942, 1 October 1985.
 
       3.   The Explanatory Supplement to the Astronomical Almanac (1992)
            Edited by P. Kenneth Seidelmann, University Science Books, Mill
            Valley, California 94941
 
       4.   SCLK Required Reading (sclk.req).
 
   The variable names used are consistent with notations used in the
   Astronomical Almanac.
 
   For a general and very accessible discussion of time we recommend:
 
       5.   James Jespersen and Jane Fitz-Randolph ``From Sundials to
            Atomic Clocks---Understanding Time and Frequency'' (Dover
            Publications, Inc. 1977) ISBN 0-486-24265-X
 
 
Introduction
--------------------------------------------------------
 
   This document describes the software available in the CSPICE Toolkit for
   manipulating various representations of time. It is your main source for
   general information about calendar based and continuous time systems in
   CSPICE . For specifics of a particular routine you should consult the
   header of that routine.
 
   In addition to the discussion of time software, there are two appendices
   to this document. The first provides basic background material on
   various time systems. The second discusses the details of how time
   strings are parsed in the CSPICE system.
 
   The Toolkit also supports conversion between spacecraft clock (SCLK) and
   Barycentric Dynamical Time (TDB). However, spacecraft clock conversion
   is mentioned only in the context of background information in Appendix
   A. CSPICE routines dealing with spacecraft clock are discussed in SCLK
   Required Reading (sclk.req).
 
 
Intended Audience
 
   This document is intended for all CSPICE users.
 
 
Detection of Non-native Text Files
 
   Starting with the N0057 release of the SPICE Toolkit (March, 2004) the
   SPICE data loading mechanism detects and prohibits loading text kernel
   files containing lines terminated with EOF character(s) non-native to
   the platform on which the Toolkit was compiled. If a non-native EOL
   terminator is detected in the first 132 characters of a text kernel, the
   execution is stopped and an error message is displayed. This feature
   does not work with files that are smaller that 132 bytes or have the
   first line longer that 132 characters.
 
 
Overview
--------------------------------------------------------
 
   CSPICE contains a versatile set of time conversion routines designed to
   simplify conversions between several time systems. The basic time
   systems supported are: Coordinated Universal Time (UTC), Barycentric
   Dynamical Time (TDB) and Terrestrial Dynamical Time (TDT). In addition,
   most common time formats are supported including: calendar, day of year,
   and Julian Date.
 
   A brief description of the various time systems is given in Appendix A.
 
 
If You're in a Hurry
 
   We'll discuss things in more detail in a moment, but in case you are
   just looking for the right name of the routine to perform some time
   transformation task, here is a classification of the time routines in
   CSPICE. We touch on only the most important routines in the remainder of
   this overview.
 
   Loading a Leapseconds Kernel
 
       furnsh_c ( filename );
 
   Converting strings to ET
 
       str2et_c ( string, et );
 
       utc2et_c ( utcstr, et);
 
       tparse_c ( string, lenout, sp2000, errmsg  )
 
   Converting ET to a string
 
       timout_c (  et, pictur, lenout, output );
 
       et2utc_c ( et, format, prec, lenout, utcstr );
 
       etcal_c  ( et, lenout, string );
 
   Converting between numeric representations of time
 
       unitim_c ( dptime, insys, outsys );
 
   Runtime modification of behavior
 
       timdef_c ( action, item, lenout, value );
 
       tsetyr_c ( year );
 
       tparch_ ( yesno, yesno_len);
 
   Formatting aid
 
       tpictr_c ( sample, lenout, lenerr, pictur, ok, error );
 
   Converting ET to local solar time on the surface of an object.
 
      et2lst_c ( et, body, lon, type, timlen, ampmlen,
                 hr, mn, sc, time, ampm );
 
   Foundation routines
 
       ttrans_ ( from, to, tvec, from_len, to_len);
 
       tpartv_ ( string, tvec, ntvec, typ, modify,
                 mods, yabbrv, succes, pictur, error,
                 string_len, typ_len, modify_len,
                 pictur_len, error_len);
 
   Utilities
 
       deltet_ ( epoch, eptype, delta, eptype_len);
 
       texpyr_ ( year );
 
       tchckd_ ( yesno, yesno_len );
 
       jul2gr_ ( year, month, day, doy );
 
       gr2jul_ ( year, month, day, doy );
 
       tcheck_ ( tvec, typ, mods, modify, ok, error,
                 typ_len, modify_len, error_len);
 
   Time constants
 
       b1900_c ();
       b1950_c ();
       j1900_c ();
       j1950_c ();
       j2000_c ();
       j2100_c ();
       jyear_c ();
       spd_c   ();
       tyear_c ();
 
 
The J2000 Epoch
 
   The basic spatial reference system for CSPICE is the J2000 system. This
   is an inertial reference frame in which the equations of motion for the
   solar system may be integrated. This reference frame is specified by the
   orientation of the earth's mean equator and equinox at a particular
   epoch --- the J2000 epoch. This epoch is Greenwich noon on January 1,
   2000 Barycentric Dynamical Time. Throughout the CSPICE documentation,
   you will see the expressions: ``seconds past 2000''; ``seconds past
   J2000''; or ``seconds past the J2000 epoch.'' In all cases, the
   reference epoch is noon January 1, 2000 on a particular time scale.
 
   (As we've just seen ``J2000'' is used to name the fundamental inertial
   frame and a particular epoch. This can sometimes be confusing if you are
   not careful to distinguish the context in which the term ``J2000'' is
   used.)
 
 
Leapseconds
 
   In almost all cases, before converting between different representations
   of time you must ``load'' a leapseconds kernel (LSK) into memory. The
   leapseconds kernel is a text kernel and is loaded via the routine
   furnsh_c.
 
       furnsh_c ( "<file name of leapseconds kernel>" );
 
   The leapseconds kernel is discussed in more detail later in this
   document.
 
 
Converting Time Strings to Numeric Representations
 
   If you are starting with a representation of time in the form of a
   string such as ``Mon Sep 30 09:59:10 PDT 1996'' you will normally need
   to get this into a numeric representation before you can work with it.
   The basic routine for converting strings to a numeric representation is
   str2et_c (``String to ET'').
 
       str2et_c ( string, et );
 
   str2et_c computes the ephemeris epoch corresponding to an input string.
   The ephemeris epoch is represented as seconds past the epoch of the
   J2000 reference frame in the time system known as Barycentric Dynamical
   Time (TDB). This time system is also referred to as Ephemeris Time (ET)
   throughout the CSPICE Toolkit.
 
   The variety of ways people have developed for representing times is
   enormous. It is unlikely that any single subroutine can accommodate all
   of the custom time formats that have arisen in various computing
   contexts. However, we believe that str2et_c correctly interprets most
   time formats used throughout the planetary science community. For
   example str2et_c supports ISO time formats, UNIX `date` output formats.
   VMS time formats, MS-DOS formats, epochs in both the A.D. and B.C. eras,
   time zones, etc.
 
   If you've been using the Toolkit for a while you are probably familiar
   with the routine utc2et_c.
 
       utc2et_c ( utcstr, et );
 
   utc2et_c provides a subset of the capabilities contained in str2et_c. It
   does not recognize time zones or time systems other than the UTC system.
   However, it has been the work horse for time conversion within the
   Toolkit for many years. In version N0046 of the Toolkit it was upgraded
   to support ISO time formats.
 
   If you are writing new code, we recommend that you use the routine
   str2et_c. There is no need to upgrade any of your existing code that
   calls utc2et_c. However, you may want to replace calls to utc2et_c with
   calls to str2et_c due to the greater flexibility of str2et_c.
 
 
Converting Numeric Representations to Time Strings
 
   If you need to examine an epoch given as some double precision number of
   seconds past J2000, you will normally want to convert it to some more
   meaningful representation. There are two routines normally used for this
   task. They offer varying degrees of flexibility in the output strings
   they can produce. The more general of these is timout_c.
 
       timout_c ( et, pictur, lenout, output );
 
   Given an epoch ET expressed as double precision seconds past J2000 and a
   format picture pictur that you would like to use as a model for the
   output time strings, timout_c produces a string representing the input
   ET in a format that matches the one specified by pictur with the length
   of the string lenout. Using timout_c you can produce a time string in
   almost any format you desire (including many that cannot be recognized
   by any of the CSPICE software). To assist in creating a format picture
   the routine tpictr_c is provided. tpictr_c takes a sample time string
   and produces the format picture that corresponds to the sample. By using
   tpictr_c and timout_c together you can easily produce strings in the
   format you are used to seeing.
 
   Less flexible, but slightly easier to use, et2utc_c has been the
   standard CSPICE time formatting routine for many years.
 
       et2utc_c ( et, format, prec, lenout, utcstr );
 
   This routine supports several fixed formats: calendar, Julian Date
   (UTC), day-of-year, ISO year/month/day, and ISO year/day-of-year. You
   may adjust the number of digits that follow the decimal point in the
   seconds component (or day in the Julian Date format).
 
 
Converting between Different Numeric Formats
 
   You may need to convert between different numeric representations of
   time such as TDT, Julian Date TDB, TAI seconds past J2000, etc. The
   routine unitim_c is available for such conversions.
 
       unitim_c ( epoch, insys, outsys );
 
 
Initialization
--------------------------------------------------------
 
 
Leapseconds Kernel
 
   Most CSPICE time routines make use of the information contained in a
   leapseconds kernel. Specifically, all of the following routines make use
   of the leapseconds kernel.
 
         str2et_c   Converts strings to ET.
 
         utc2et_c   Converts UTC strings to ET
 
         timout_c   Converts ET to strings
 
         et2utc_c   Converts ET to a UTC string.
 
         unitim_c   Converts between numeric time systems
 
         ttrans_    Converts between different parsed representations of
                    time
 
   Before any of these routines can be used you must ``load'' a leapseconds
   kernel into the ``kernel pool.'' This is done by calling the routine
   furnsh_c, whose calling sequence is:
 
       furnsh_c ( kernel );
 
   `kernel' is the name of a ``leapseconds kernel.'' Leapseconds kernels
   are text based kernels containing the epochs of leap seconds and other
   constants required by the time conversion routines.
 
   The leapseconds kernel needs to be loaded just once per program run;
   normally, the leapseconds kernel is loaded in a program's initialization
   section.
 
   The precise contents of the leapseconds kernel are discussed in the
   section ``Computing Delta ET'' below. Text kernels and the routine
   furnsh_c are discussed in more detail in KERNEL Required Reading,
   kernel.req.
 
 
SPK and PCK kernels
 
   The routine et2lst_c converts ephemeris time (ET) to the local solar
   time for a point at a user specified longitude on the surface of a body.
   This computation is performed using the bodyfixed location of the sun.
   Consequently, to use et2lst_c you must first load SPK and PCK files that
   contain sufficient position and orientation data for the computation of
   the bodyfixed location of the sun.
 
   SPK files are loaded using the routine furnsh_c.
 
       furnsh_c ( "<spk file name>" );
 
   PCK files are usually text based. Text based kernels are loaded by
   calling furnsh_c.
 
       furnsh_c ( kernel );
 
   Occasionally, PCK, files are binary (DAF based) files that contain the
   orientation of an object with respect to an inertial frame. Binary PCK
   files are loaded with the routine furnsh_c.
 
       furnsh_c ( "<binary pck file name>" );
 
   As with the leapseconds kernel, SPK and PCK files need to be loaded just
   once per program run---usually at program initialization.
 
 
Input String Conversion
--------------------------------------------------------
 
   We normally represent epochs as a combination of a date and time of day.
   The simplest means of specifying an epoch as a date and time is to
   create a string such as:
 
       SpiceChar * string = "Oct 1, 1996 09:12:32";
 
   However, arithmetic is most easily performed with numeric
   representations of time. In CSPICE we represent epochs as some number of
   double precision seconds past the J2000 epoch.
 
   CSPICE contains three routines for converting strings directly to
   ``seconds past 2000.'' They are str2et_c, utc2et_c, and tparse_c. All of
   these routines take a string as input and produce a double precision
   number that gives the number of seconds past the J2000 epoch
   corresponding to the input string. The method of analyzing the input
   string and assigning meaning to its various components is identical for
   all three routines. This analysis is called ``parsing'' the string. All
   three routines, str2et_c, utc2et_c and tparse_c, use the ``foundation''
   routine tpartv_ to parse the input string. Each then interprets the
   results of tpartv_ to assign meaning to the string. Below are a number
   of examples of strings and the interpretation assigned to the various
   components.
 
   ISO (T) Formats.
 
      String                        Year Mon  DOY DOM  HR Min Sec
      ----------------------------  ---- ---  --- ---  -- --- ------
      1996-12-18T12:28:28           1996 Dec   na  18  12  28 28
      1986-01-18T12                 1986 Jan   na  18  12  00 00
      1986-01-18T12:19              1986 Jan   na  18  12  19 00
      1986-01-18T12:19:52.18        1986 Jan   na  18  12  19 52.18
      1995-08T18:28:12              1995  na  008  na  18  28 12
      1995-18T                      1995  na  018  na  00  00 00
 
   Calendar Formats.
 
      String                        Year   Mon DOM  HR Min  Sec
      ----------------------------  ----   --- ---  -- ---  ------
      Tue Aug  6 11:10:57  1996     1996   Aug  06  11  10  57
      1 DEC 1997 12:28:29.192       1997   Dec  01  12  28  29.192
      2/3/1996 17:18:12.002         1996   Feb  03  17  18  12.002
      Mar 2 12:18:17.287 1993       1993   Mar  02  12  18  17.287
      1992 11:18:28  3 Jul          1992   Jul  03  11  18  28
      June 12, 1989 01:21           1989   Jun  12  01  21  00
      1978/3/12 23:28:59.29         1978   Mar  12  23  28  59.29
      17JUN1982 18:28:28            1982   Jun  17  18  28  28
      13:28:28.128 1992 27 Jun      1992   Jun  27  13  28  28.128
      1972 27 jun 12:29             1972   Jun  27  12  29  00
      '93 Jan 23 12:29:47.289       1993*  Jan  23  12  29  47.289
      27 Jan 3, 19:12:28.182        2027*  Jan  03  19  12  28.182
      23 A.D. APR 4, 18:28:29.29    0023   Apr  04  18  28  29.29
      18 B.C. Jun 3, 12:29:28.291   -017   Jun  03  12  29  28.291
      29 Jun  30 12:29:29.298       2029+  Jun  30  12  29  29.298
      29 Jun '30 12:29:29.298       2030*  Jun  29  12  29  29.298
 
   Day of Year Formats
 
      String                        Year  DOY HR Min Sec
      ----------------------------  ----  --- -- --- ------
      1997-162::12:18:28.827        1997  162 12  18 28.827
      162-1996/12:28:28.287         1996  162 12  28 28.287
      1993-321/12:28:28.287         1993  231 12  28 28.287
      1992 183// 12 18 19           1992  183 12  18 19
      17:28:01.287 1992-272//       1992  272 17  28 01.287
      17:28:01.282 272-1994//       1994  272 17  28 01.282
      '92-271/ 12:28:30.291         1992* 271 12  28 30.291
      92-182/ 18:28:28.281          1992* 182 18  28 28.281
      182-92/ 12:29:29.192          0182+ 092 12  29 29.192
      182-'92/ 12:28:29.182         1992  182 12  28 29.182
 
   Julian Date Strings
 
      jd 28272.291                  Julian Date   28272.291
      2451515.2981 (JD)             Julian Date 2451515.2981
      2451515.2981 JD               Julian Date 2451515.2981
 
   Abbreviations Used in Tables
 
      na    --- Not Applicable
      Mon   --- Month
      DOY   --- Day of Year
      DOM   --- Day of Month
      Wkday --- Weekday
      Hr    --- Hour
      Min   --- Minutes
      Sec   --- Seconds
 
         *   The default interpretation of a year that has been abbreviated
             with a leading quote as in 'xy (such as '92) is to treat the
             year as 19xy if xy is more than 49 and to treat it is 20xy
             otherwise. Thus '52 is interpreted as 1952 and '47 is treated
             as 2047.
 
         +   When a day of year format or calendar format string is input
             and neither of the integer components of the date is greater
             than 1000, the first integer is regarded as being the year.
 
 
Parsing Time Strings
 
   A time string is parsed by first scanning the string from left to right
   and identifying recognizable substrings. (integers, punctuation marks,
   names of months, names of weekdays and time systems, time zones, etc.)
   These recognizable substrings are called the tokens of the input string.
   The meaning of some tokens are immediately determined. For example named
   months, weekdays and time systems have clear meanings. However, the
   meanings of numeric components must be deciphered from their magnitudes
   and location in the string relative to the immediately recognized
   components of the input string.
 
   The following substrings are immediately recognizable.
 
       1.   All months (January, February, ... ) or any abbreviation of at
            least 3 letters;
 
       2.   All weekdays (Sunday, Monday, ... ) or any abbreviation of at
            least 3 letters;
 
       3.   Standard abbreviations of U.S. time zones: "EST", "EDT", "CST",
            "CDT", "MST", "MDT", "PDT", "PST".
 
       4.   The abbreviations for eras: "B.C.", "BC", "A.D.", and "AD";
 
       5.   Time systems: "TDT", "TDB", "UTC" (Note that "ET" is not a
            recognized time system);
 
       6.   Julian Date Label: "JD" (Note that JED is not a recognized
            Julian Date Label);
 
       7.   The 12-hour clock labels: "A.M.", "AM", "P.M." and "PM";
 
       8.   Time Zones expressed as UTC offsets: UTC+HR:MN, UTC-HR:MN where
            HR is an unsigned integer between 0 and 12 inclusive; MN is an
            unsigned integer between 0 and 59 inclusive.
 
   With the exception of months, all items above may be enclosed in
   parentheses. For example "TDB" and "(TDB)" are both recognized as the
   same time system.
 
   The case of the letters in these substrings does not matter. For example
   all of the various ways of writing "TDB" ( "TDB", "tDB", ... "tdb") are
   recognized as "TDB".
 
   It is not necessary to leave space between the various substrings. For
   example JDTDT and JDUTC are recognized as "JD" followed by "TDT" and
   "JD" followed by "UTC" respectively.
 
   To determine the meaning of the numeric tokens in the input string, a
   set of transformation rules are applied to the full set of tokens in the
   string. These transformations are repeated until the meaning of every
   token has been determined or until further transformations yield no new
   clues into the meaning of the numeric tokens. Here is an overview of the
   rules that are applied to the various tokens in the string.
 
       1.   Unless the substring JD or jd is present the string is assumed
            to be a calendar format (day-month-year or year and day of
            year). If the substring JD or jd is present, the string is
            assumed to represent a Julian date.
 
       2.   If the Julian date specifier is not present, any integer
            greater than 999 is regarded as being a year specification.
 
       3.   A dash `-' can represent a minus sign only if it precedes the
            first digit in the string and the string contains the Julian
            date specifier (JD). (No negative years, months, days, etc are
            allowed).
 
       4.   Numeric components of a time string must be separated by a
            character that is not a digit or decimal point. Only one
            decimal component is allowed. For example 1994219.12819 is
            sometimes interpreted as the 219th day of 1994 + 0.12819 days.
            The CSPICE time parsing software does not support such strings.
 
       5.   No exponential components are allowed. For example you can't
            input 1993 Jun 23 23:00:01.202E-4. You have to explicitly list
            all zeros that follow the decimal point: i.e. 1993 Jun 23
            23:00:00.0001202
 
       6.   The single colon (:) when used to separate numeric components
            of a string is interpreted as separating Hours, Minutes, and
            Seconds of time.
 
       7.   If a double slash (//) or double colon (::) follows a pair of
            integers, those integers are assumed to represent the year and
            day of year.
 
       8.   A quote followed by an integer less than 100 is regarded as an
            abbreviated year. For example: '93 would be regarded as the
            93rd year of the reference century. See texpyr_ for further
            discussion of abbreviated years.
 
       9.   An integer followed by "B.C." or "A.D." is regarded as a year
            in the era associated with that abbreviation.
 
      10.   All dates are regarded as belonging to the extended Gregorian
            Calendar (the Gregorian calendar is the calendar currently used
            by western society).
 
      11.   If the ISO date-time separator (T) is present in the string,
            only ISO allowed token patterns are examined for a match with
            the current set of tokens. If no match is found the search is
            abandoned and appropriate diagnostic messages are generated.
 
      12.   If two delimiters are found in succession in the time string,
            the time string is diagnosed as an erroneous string.
            (Delimiters are comma, white space, dash, slash, period, day of
            year mark)
 
            Note the delimiters do not have to be the same. The pair of
            characters ``,-'' counts as two successive delimiters.
 
      13.   White space and commas serve only to delimit tokens in the
            input string; they do not affect the meaning of any of the
            tokens.
 
      14.   When the sizes of the integer components do not clearly specify
            a year but the name of a month is present (for example "APR")
            the following patterns are assumed
 
                  Year Month Day
                  Month Day Year
                  Year Day Month
 
      15.   When integer components are separated by slashes (/) as in
            3/4/5. The integers are assumed to be Month, Day, Year. Thus in
            our example "3/4/5" is assumed to mean 4th of March in the year
            '05.
 
      16.   If a day of year marker is present (// or ::) and the size of
            the integer components does not clearly specify the year (as in
            45-33//) the string is interpreted as Year Day-of-Year. Thus
            45-33// is interpreted as the 33rd day of the year '45.
 
   Once the various tokens have been determined and a meaning attached to
   them, the routines str2et_c, utc2et_c, and tparse_c use the tokens to
   construct the double precision number giving the number of seconds past
   J2000 that corresponds to input string. However, not all tokens or token
   combinations are allowed by the routines.
 
 
str2et_c
--------------------------------------------------------
 
   The routine str2et_c is the most flexible time transformation routine.
   str2et_c accepts the widest variety of time strings. To illustrate the
   various features of str2et_c we begin by considering the string
 
      1988 June 13, 3:29:48
 
   There is nothing in this string to indicate what time system the date
   and time belong to. Moreover, there is nothing to indicate whether the
   time is based on a 24-hour clock or twelve hour clock.
 
   In the absence of such indicators, the default interpretation of this
   string is to regard the time of day to be a time on a 24-hour clock in
   the UTC time system. The date is a date on the Gregorian Calendar (this
   is the calendar used in nearly all western societies).
 
 
Labels (A.M. and P.M.)
 
   If you add more information to the string, str2et_c can then make a more
   informed interpretation of the time string. For example:
 
      1988 June 13, 3:29:48 P.M.
 
   is still regarded as a UTC epoch. However, with the addition of the
   ``P.M.'' label it is now interpreted as the same epoch as the unlabeled
   epoch 1988 June 13, 15:29:48. Similarly
 
      1988 June 13, 12:29:48 A.M.
 
   is interpreted as
 
      1988 June 13, 00:29:48
 
   on the 24-hour clock.
 
 
For the Record
 
   12:00 A.M. corresponds to Midnight (00:00 on the 24-hour clock). 12:00
   P.M. corresponds to Noon (12:00 on the 24-hour clock).
 
 
Labels (Time Zones)
 
   You may add still further indicators to the string. For example
 
      1988 June 13, 3:29:48 P.M. PST
 
   is interpreted as an epoch in the Pacific Standard Time system. This is
   equivalent to
 
      1988 June 13, 23:29:48 UTC
 
   All of the standard abbreviations for U.S. time zones are recognized by
   the time parser.
 
      EST   --- Eastern Standard Time  ( UTC-5:00 )
      CST   --- Central Standard Time  ( UTC-6:00 )
      MST   --- Mountain Standard Time ( UTC-7:00 )
      PST   --- Pacific Standard Time  ( UTC-8:00 )
 
      EDT   --- Eastern Daylight Time  ( UTC-4:00 )
      CDT   --- Central Daylight Time  ( UTC-5:00 )
      MDT   --- Mountain Daylight Time ( UTC-6:00 )
      PDT   --- Pacific Daylight Time  ( UTC-7:00 )
 
   In addition, any other time zone may be specified by representing its
   offset from UTC.
 
   To specify an offset from UTC you need to create an offset label. The
   label starts with the letters `UTC' followed by a `+' for time zones
   east of Greenwich and `-' for time zones west of Greenwich. This is
   followed by the number of hours to add or subtract from UTC. This is
   optionally followed by a colon `:' and the number of minutes to add or
   subtract to get the local time zone. Thus to specify the time zone of
   Calcutta (which is 5 and 1/2 hours ahead of UTC) you would specify the
   time zone to be UTC+5:30. To specify the time zone of Newfoundland
   (which is 3 and 1/2 hours behind UTC) use the offset notation UTC-3:30.
 
 
For the Record
 
   Leapseconds occur at the same time in all time zones. In other words,
   the seconds component of a time string is the same for any time zone as
   is the seconds component of UTC. The following are all legitimate ways
   to represent an epoch of some event that occurred in the leapsecond
 
      1995 December 31 23:59:60.5  (UTC)
 
 
      1996 January   1, 05:29:60.5  (UTC+5:30 --- Calcutta Time)
      1995 December 31, 20:29:60.5  (UTC-3:30 --- Newfoundland)
      1995 December 31  18:59:60.5  (EST)
      1995 December 31  17:59:60.5  (CST)
      1995 December 31  16:59:60.5  (MST)
      1995 December 31  15:59:60.5  (PST)
 
 
Labels ( TDT, TDT, and UTC )
 
   In addition to specifying time zones you may specify that the string be
   interpreted as a formal calendar representation in either the
   Barycentric Dynamical Time system (TDB) or the Terrestrial Dynamical
   Time system (TDT).
 
   In these systems there are no leapseconds; every day has exactly 86400
   seconds. TDB times are written as
 
      1988 June 13, 12:29:48 TDB
 
   TDT times are written as:
 
      1988 June 13, 12:29:48 TDT
 
   To add clarity or to override any changes you happen to make to the
   default behavior of ET2STR (see below) you may add the label ``UTC'' to
   any time string.
 
      1998 Jun 13, 12:29:48 UTC
 
   Note that the system label may be placed anywhere in the time string.
   All of the following will be understood by the time parsing software:
 
      TDB 1988 June 13, 12:29:48
      1988 June 13, 12:29:48 TDB
      1988 June 13, TDB 12:29:48
 
 
utc2et_c
--------------------------------------------------------
 
   The routine utc2et_c can be thought of as a version of str2et_c that
   allows a narrower range of inputs. It converts strings in the UTC system
   to TDB seconds past the J2000 epoch. It does not support other time
   systems or time zones. In addition utc2et_c does not recognize times on
   a 12-hour clock. Strings such as
 
      1983 June 13, 9:00:00 A.M.
 
   are treated as erroneous by utc2et_c.
 
 
tparse_c
--------------------------------------------------------
 
   The routine tparse_c can be thought of as a narrow version of str2et_c
   that allows only TDB as input. tparse_c converts strings on a formal
   time scale to seconds past the J2000 epoch. tparse_c doesn't ``know''
   anything about leapseconds. Since tparse_c does not make use of
   leapseconds, it can be used without first loading a leapseconds kernel.
 
   Like utc2et_c, tparse_c does not recognize other time systems or time
   zones. Also it does not recognize times on a 12-hour clock.
 
   Unlike str2et_c and utc2et_c, tparse_c does not make use of the CSPICE
   exception handling subsystem. Erroneous strings are diagnosed via a
   string---`error'. If the string `error' is returned empty (blank) no
   problems were detected in the input string. If `error' is returned by
   tparse_c non-blank, it contains a diagnostic message that indicates
   problems with the input time string.
 
 
Changing Default Behavior
--------------------------------------------------------
 
   The three time string transformation routines can be adjusted at run
   time so that various built in defaults can be changed without re-writing
   any of the code for the routines.
 
 
Abbreviated Years
 
   All three string transformation routines treat abbreviated years in the
   same fashion. The default behavior is to map any abbreviated year into
   the range from 1968 to 2067. Thus the year 22 corresponds to 2022; 77
   corresponds to 1977. However, you may reset the lower end of this 100
   year range via the routine tsetyr_c. For example if you would like to
   set the default range to be from 1972 to 2071 issue the following
   subroutine call:
 
       tsetyr_c ( 1972 );
 
   Note that this change affects the behavior of all three string
   conversion routines.
 
 
Range of Time String Components
 
   The routines tparse_c and utc2et_c accept time strings whose numeric
   components are outside of the normal range of values used in time and
   calendar representations. For example strings such as
 
      1985 FEB 43 27:65:25  (equivalent to 1985 MAR 16 04:05:25)
 
   will be accepted as input. You might wish to restrict the range of input
   strings so that this behavior is not allowed. The routine tparch_ is
   provided for this purpose. If you place the following subroutine call
 
       tparch_ ( "YES", 3 );
       where 3 is the string length of YES.
 
   early in your program, prior to any calls to utc2et_c or tparse_c, the
   components of calendar strings will be restricted so that all calendar
   components will be in the ``expected'' range. (The exact ranges for the
   components are spelled out in the header for tparch_ )
 
   str2et_c does not accept time strings whose components are outside the
   normal range used in conversation. You cannot alter this behavior
   without re-coding str2et_c.
 
 
Default Time Systems and Time Zone
 
   When a string is presented without a time system or time zone label
   str2et_c assumes that the string represents a time in a default time
   zone or time system. If you take no action, the default time system is
   UTC. (There is no time zone offset; UTC is the same as UTC+00:00) You
   can override the default by simply including the time zone or time
   system of interest in the input time string. However, under some
   circumstances you may find that you almost always use the TDB time
   system. In such a case you would normally need to include the TDB label
   in the time string every time you use str2et_c. Hence, the defaults used
   by str2et_c might be a hindrance rather than a convenience. With this
   possibility in mind, str2et_c has been designed so that you may alter
   its default behavior with regard to default time system or time zone. To
   change the default time system or time zone use the routine timdef_c.
 
   (Keep in mind that if you specify a time zone or time system label in
   the input time string the default time zone or system is not used. The
   label in the string is used to determine the time zone or time system.)
 
 
Changing the Time System
 
   Three time systems are supported: UTC, TDB, TDT. To change the default
   system to one of these three systems issue the appropriate subroutine
   call below:
 
       timdef_c ( "SET", "SYSTEM", lenout, "UTC" );
       timdef_c ( "SET", "SYSTEM", lenout, "TDB" );
       timdef_c ( "SET", "SYSTEM", lenout, "TDT" );
 
   Note that setting a time system turns off any default time zone you may
   have set using timdef_c.
 
 
Time Zones
 
   All time zones are supported by str2et_c. The default time zone is
   simply Greenwich Mean Time (UTC+00:00). To change the default behavior
   of str2et_c so that unlabeled strings are assumed to be referenced to a
   particular time zone (for example Pacific Standard Time) issue the
   subroutine call below:
 
       timdef_c ( "SET", "ZONE", lenout, "PST" );
 
   Note that setting a time zone turns off any default time system you may
   have set via timdef_c.
 
 
Calendars
 
   The default calendar used by str2et_c is the Gregorian calendar.
   However, the Gregorian calendar did not come into existence until
   October 15, 1582. To complicate matters, many countries did not adopt
   the Gregorian calendar until centuries later. Prior to adoption of the
   Gregorian calendar most western societies used the Julian calendar. The
   generation of successive days is identical on the Julian and Gregorian
   calendars except for the determination of leap days in the last year of
   a century such as the year 1900. On the Julian calendar, a leap day is
   inserted as the last day of February every 4 years. on the Gregorian
   calendar, a leap day is inserted as the last day of February every 4
   years with the possible exception of the last year of a century (such as
   1900). The last year of a century is a leap year only if the year is
   evenly divisible by 400. Thus the year 2000 is a leap year on the
   Gregorian calendar but 1900 is not.
 
   Both the Gregorian and Julian calendars can be extended forward and
   backward in time indefinitely. The default behavior of str2et_c is to
   use the Gregorian calendar for all epochs. However, using timdef_c you
   can set the default calendar to one of three: GREGORIAN, JULIAN, or
   MIXED:
 
       timdef_c ( "SET", "CALENDAR", lenout, "GREGORIAN" );
       timdef_c ( "SET", "CALENDAR", lenout, "JULIAN"    );
       timdef_c ( "SET", "CALENDAR", lenout, "MIXED"     );
 
   The ``MIXED'' calendar assumes that calendar strings for epochs prior to
   October 5, 1582 belong to the Julian Calendar; strings for later epochs
   are assumed to belong to the Gregorian Calendar. The specification of a
   calendar, does not affect a previous setting of a time system or time
   zone. You can change the calendar used by str2et_c only through the
   routine timdef_c; there are no labels recognized by str2et_c for the
   various calendars.
 
 
Output Conversion
--------------------------------------------------------
 
   Times need to be printed out as well as read in. CSPICE contains three
   routines for accomplishing this task: timout_c, et2utc_c, and etcal_c.
   All three convert a number of ephemeris seconds past J2000 to a time
   string.
 
 
timout_c
 
   The routine timout_c provides a mechanism for formatting output time
   strings in almost any form you desire. The calling sequence for timout_c
   is:
 
       timout_c ( et, pictur, lenout, output );
 
   where
 
         `et'        is a double precision number containing the number of
                     TDB seconds past J2000 corresponding to some epoch.
 
         `pictur'    is a characters string that describes how the output
                     string should be formatted. It is a ``picture'' of the
                     format for output.
 
         lenout      is the user specified length of the string output.
 
         `output'    is the string corresponding to `et' and `pictur'.
 
   To see how this works, consider the following example time string:
 
      04:29:29.292 Jan 13, 1996
 
   The value of `pictur' to use to create time strings that are similar in
   appearance to the example string is:
 
      SpiceChar * pictur = "HR:MN:SC.### Mon DD, YYYY ::RND";
 
   Most of this components in `pictur' are fairly obvious. The exception is
   the substring
 
      "::RND".
 
   This substring tells timout_c to round the seconds portion of the output
   string instead of simply truncating. (Note that the case of the letters
   is significant in pictur.) timout_c can produce strings representing
   epochs in the time systems (UTC, TDB, TDT) or any time zone, and on
   either the Julian, Gregorian Calendar or Mixed Calendar. You may round
   or truncate numeric components.
 
   The rules for constructing pictur are spelled out in the header to
   timout_c. However, you may very well never need to learn these rules.
   CSPICE contains the routine tpictr_c that can construct a time format
   picture for you from a sample time string. Returning to the example
   above, if the following block of code is executed, pictur will contain
   the format picture that will yield output strings similar to our example
   string.
 
       SpiceChar * exampl = "04:29:29.292 Jan 13, 1996";
 
       tpictr_c ( exampl, lenout, lenerr, pictur, ok, error );
 
   The arguments ok and error are outputs from tpictr_c. They are present
   because some strings are not recognized as time strings. tpictr_c
   recognizes the same set of time strings as do str2et_c, utc2et_c and
   tparse_c. However, if you want your output string to be in a system
   other than UTC you must supply the label for that system in your example
   string. tpictr_c can construct format pictures for strings that are not
   accepted by the string conversion routines. For example, if you would
   like to suppress the year in a calendar output format, you could use the
   following example string:
 
       SpiceChar * exampl = "Jan 12, 02:28:29.### A.M. (PDT)";
 
   Even though this string is ambiguous as an epoch (there's no year
   specified), it is sufficient for determining a picture that describes
   its format. If you decide to use tpictr_c with inputs like this, be sure
   to check the output flag OK. Even though you know what is intended,
   tpictr_c may have problems with some ambiguous time strings.
 
 
et2utc_c
 
   The routine et2utc_c is an older time formatting routine. It is not as
   flexible as timout_c. All outputs are UTC outputs and only a limited set
   of formats are supported. On the other hand it is easier to learn how to
   use et2utc_c. et2utc_c is an inverse to utc2et_c: that is following the
   calls
 
       utc2et_c ( utcin,  et                     );
       et2utc_c ( et ,   "C",  3, lenout, utcout );
 
   utcout is identical in content to (although probably formatted
   differently from) UTCIN. et2utc_c can create time strings in any of the
   following formats.
 
      Format      Name            Example
      ------      -----------     --------------------------
      "C"         Calendar        "1979 JUL 04 14:19:57.184"
      "D"         Day of Year     "1979-114 // 14:19:57.184"
      "J"         Julian Date     "JD 2433282.529"
      "ISOC"      ISO Calendar    "1987-04-12T16:31:12.814"
      "ISOD"      ISO Day of Year "1987-102T16:31:12.814"
 
   In addition, you may specify the number of decimal places in the
   fractional part of the seconds token or the Julian Date (three, in the
   examples above). Note that Julian Dates are prefaced with the character
   string `JD' (and are UTC Julian Dates). This allows strings generated by
   et2utc_c to be used later as inputs to utc2et_c or str2et_c.
 
 
etcal_c
 
   The routine etcal_c is a utility routine. It can produce outputs in a
   single format with a fixed number of decimal places. Moreover, the
   calendar strings it produces are on a formal calendar. There are no
   leapseconds; each day has exactly 86400 seconds. Since it does not make
   use of leapseconds, you don't need to load a leapseconds kernel prior to
   calling etcal_c. This makes it well suited for producing diagnostic
   messages. Indeed, it was created so that more user friendly diagnostic
   messages could be produced by those SPICE routines that require ET as an
   input.
 
 
Converting Between Uniform Time Scales
--------------------------------------------------------
 
   We use the term uniform time scale to refer to those representations of
   time that are numeric (each epoch is represented by a number) and
   additive. A numeric time system is additive if given the representations
   E1 and E2 of any pair of successive epochs, the time elapsed between the
   epochs is given by the difference E2 - E1.
 
   Conversion between uniform time scales can be carried out via the double
   precision function unitim_c. The uniform time scales that are supported
   by this routine are:
 
      "TAI"     International Atomic Time.
      "TDB"     Barycentric Dynamical Time.
      "TDT"     Terrestrial Dynamical Time.
      "ET"      Ephemeris time
      "JDTDB"   Julian Date relative to TDB.
      "JDTDT"   Julian Date relative to TDT.
      "JED"     Julian Ephemeris date.
 
      *  In the @SPICE system ET  is synonymous to TDB.
      ** In the @SPICE system JED is synonymous to JDTDB.
 
 
Local Solar Time
--------------------------------------------------------
 
   Local solar time is a used to give people an idea of how high the sun is
   in the sky as seen from a particular site on surface of a planet or
   satellite. When the Sun is on the zenith meridian, the local solar time
   is 12:00:00 noon. For points on the equator of a body, the Sun rises
   around 6:00:00 A.M. local solar time; it sets around 6:00:00 P.M. local
   solar time.
 
   Formally, the local solar time at a site on a body is the difference
   between the planetocentric longitude of the site and the planetocentric
   longitude of the Sun as seen from the center of the body. The angular
   difference in these two longitudes is measured in hours, minutes, and
   seconds in the same sense that hours, minutes and seconds are used to
   measure right ascension--- 24 hours in 360 degrees; 60 minutes in an
   hour; 60 seconds in a minute. When the sun in on the zenith meridian the
   hour is defined to be 12. Finally, the hours increase from sunrise to
   sunset.
 
   Because of these conventions, an hour of local solar time will not be of
   the same duration as a UTC hour. In the case of a site on Mars, a solar
   hour will be approximately 62 UTC minutes.
 
   Local solar time for a specific site can be computed using the routine
   et2lst_c (ET to Local Solar Time).
 
 
Foundation Routines and Utilities
--------------------------------------------------------
 
   At the heart of the CSPICE time software subsystem are the
   ``foundation'' routines tpartv_ and ttrans_. tpartv_ is used to take
   apart a time string and convert it to a vector of numeric components.
   ttrans_ serves the role of converting between the various numeric vector
   representations of time. If you need to build your own time conversion
   routines, these routines are a good place to begin.
 
   In addition to the foundation routines, you may find helpful the
   following utility routines.
 
         texpyr_    converts two-digit abbreviated years to full years. You
                    set lower bound of the 100 year mapping interval via
                    the routine tsetyr_c discussed earlier in this
                    document.
 
         tcheck_    takes a numeric vector representing the components of a
                    calendar time and checks that all components are within
                    the normal range used in conversation. Note that
                    tcheck_ performs no action until you call tparch_ with
                    an argument of "YES".
 
         tchckd_    allows you to determine if component checking has been
                    enabled in tcheck_ via a call to tparch_.
 
         jul2gr_    converts the year, month, and day of an epoch on the
                    Julian Calendar to the corresponding year, month, day
                    and day-of-year on the Gregorian calendar.
 
         gr2jul_    converts the year, month, and day of an epoch on the
                    Gregorian Calendar to the corresponding year, month,
                    day and day-of-year on the Julian calendar.
 
         deltet_c   computes the time difference TDB - UTC.
 
         b1900_c    returns the Julian ephemeris date (TDB) of the epoch of
                    the Besselian date 1900.
 
         b1950_c    returns the Julian ephemeris date (TDB) of the epoch of
                    the Besselian date 1950.
 
         j1900_c    returns the Julian Date of 1899 DEC 31 12:00:00 (TDB)
 
         j1950_c    returns the Julian ephemeris date of the epoch 1 Jan
                    1950 00:00:00 (TDB).
 
         j2000_c    returns the Julian ephemeris date of the epoch 1 Jan
                    2000 12:00:00 (TDB).
 
         j2100_c    returns the Julian ephemeris date of the epoch 1 Jan
                    2100 12:00:00
 
         jyear_c    returns the number of seconds in a Julian year (365.25
                    Julian days).
 
         spd_c      returns the number of TDB seconds in a Julian day TDB
                    (86400 seconds).
 
         tyear_c    returns the number of seconds in a tropical year
                    (approximately the number of seconds from one spring
                    equinox to the next)
 
 
Example
--------------------------------------------------------
 
   The following program demonstrates use of the time conversion routines
   str2et_c, tpictr_c, timout_c and et2utc_c.
 
   Note that the data necessary to convert between UTC and ET are loaded
   into the kernel pool just once---typically during program
   initialization--- after which the conversion may be performed at any
   level within the program.
 
      /*
         Convert between UTC and ET interactively, and convert ET
         back to UTC in calendar format, DOY format, and as a
         Julian date.
 
         Requires a leapseconds kernel.
      */
 
      #include <stdio.h>
      #include "SpiceUsr.h"
 
      #define UTCLEN 34
 
 
      void main ()
      {
          SpiceDouble      et;
          SpiceChar        utcstr[UTCLEN];
          SpiceChar        dutcstr[UTCLEN];
          SpiceChar        jutcstr[UTCLEN];
          SpiceChar     *  leap;
          SpiceChar     *  response;
          SpiceInt         response_len = 50;
 
 
 
          /* Get the name of the kernel file. */
 
          leap = prompt_c ( "Name of leapsecond kernel? " );
 
 
          /* Load the kernel pool.*/
 
          furnsh_c ( leap );
 
          response = "Y";
 
 
 
          /* Compute result for each new time epoch. */
 
          do
             {
             response = prompt_c ( "Enter time string: " );
 
             str2et_c ( response, &et );
             printf( "Time converts to ET (sec past J2000) %f\n\n",
                                                               et );
 
 
             /* Convert from et to string */
 
             et2utc_c (  et , "C", 3, UTCLEN, utcstr  );
             et2utc_c (  et , "D", 3, UTCLEN, dutcstr );
             et2utc_c (  et , "J", 3, UTCLEN, jutcstr );
             printf( "ET converts to %s, %s, %s\n\n", utcstr,
                                                      dutcstr,
                                                      jutcstr );
 
 
             response = prompt_c ( "Continue? (Y/N)" );
 
             }
 
          while ( *response == 'Y' || *response == 'y' );
 
      }
 
 
Appendix A. Background Material
===========================================================================
 
   The Toolkit directly supports three time systems. They are
 
       1.   Coordinated Universal Time (UTC)
 
       2.   Barycentric Dynamical Time (TDB) also called Ephemeris Time
            (ET)
 
       3.   Spacecraft Clock Time (SCLK---pronounced ``ess clock'')
 
 
Coordinated Universal Time (UTC)
--------------------------------------------------------
 
 
International Atomic Time (TAI)
 
   Before discussing Coordinated Universal Time we feel it is helpful to
   talk about International Atomic Time (TAI or atomic time). Atomic time
   is based upon the atomic second as defined by the ``oscillation of the
   undisturbed cesium atom.'' Atomic time is simply a count of atomic
   seconds that have occurred since the astronomically determined instant
   of midnight January 1, 1958 00:00:00 at the Royal Observatory in
   Greenwich, England. Atomic time is kept by the International Earth
   Rotation Service (IERS, formally the Bureau International L'Heure---BIH)
   in Paris, France. The National Bureau of Standards and the U.S. Naval
   Observatory set their clocks by the clock maintained by the IERS.
 
 
Naming the seconds of TAI --- UTC
 
   Coordinated Universal Time is a system of time keeping that gives a name
   to each instant of time of the TAI system. These names are formed from
   the calendar date and time of day that we use in our daily affairs. They
   consist of 6 components: year, month, day, hour, minutes and seconds.
   The year, month and day components are the normal calendar year month
   and day that appear on wall calendars. The hours component may assume
   any value from 0 through 23. The minutes component may assume any value
   from 0 to 59. The seconds will usually (but not always) range from 0 to
   59.999... . The hour-minute-second string
 
      "00:00:00"
 
   is midnight and is the first instant of the calendar day specified by
   the first three components of the UTC time.
 
   In the CSPICE system UTC times are represented by character strings.
   These strings contain: year, month, day, hour, minute and second
   separated by delimiters (spaces or punctuation marks). The various
   delimiters and substrings between the delimiters are called the tokens
   of the string. A typical time string looks like
 
          "5 OCTOBER 1986 7:20:16.122 (UTC)"
 
   The tokens of the string and the associated UTC time components are
 
      "5"       --- day
      "OCTOBER" --- month
      "1986"    --- year
      "7"       --- hours
      "20"      --- minutes
      "16.122"  --- seconds
 
   The link between any token and its corresponding UTC component is
   determined by examining the values of the tokens and comparing them to
   the other tokens. The precise rules used are spelled out in great detail
   in Appendix B. For now, simply be assured that the following five
   strings all mean the same thing and are interpreted in the same way by
   CSPICE Toolkit software.
 
      "5 OCTOBER 1986"
      "1986 OCTOBER 5"
      "1986 5 OCTOBER"
      "1986 10 5"
      "10 5 1986"
 
 
Tying UTC to the Earth's Rotation
 
   The names given to TAI instants by the UTC system are governed by the
   earth's rotation. Ideally, UTC strings having hours, minutes and seconds
   components all zero should correspond to Greenwich midnight as
   determined by the observations of the transits of stars (the time system
   known as UT1). However, since the rotation of the earth is not uniform,
   this ideal cannot be realized. The difference between Greenwich midnight
   observed astronomically and UTC midnight is almost never zero. However,
   to keep the difference from becoming too large, UTC is occasionally
   adjusted so that the difference between the two midnights never exceeds
   .9 seconds. Thus from a knowledge of UTC one can always compute UT1 to
   better than 1 second accuracy.
 
 
Leapseconds
 
   When Greenwich UT1 midnight lags behind UTC midnight by more than 0.9
   seconds the IERS will announce that a leap second will be added to the
   collection of UTC names. This leap second has traditionally been added
   after the last ``normal'' UTC name of December 31 or June 30. Thus when
   a UTC second is added the hours-minutes-seconds portion of the UTC name
   progresses as shown here
 
      ... DECEMBER 31 23:59:57
      ... DECEMBER 31 23:59:58
      ... DECEMBER 31 23:59:59
      ... DECEMBER 31 23:59:60
      ... JANUARY   1 00:00:00
 
   instead of the usual progression
 
      ... DECEMBER 31 23:59:57
      ... DECEMBER 31 23:59:58
      ... DECEMBER 31 23:59:59
      ... JANUARY   1 00:00:00
 
   Should Greenwich UT1 midnight run ahead of UTC midnight by more than 0.9
   seconds the IERS will announce a negative leap second. In this case one
   of the usual UTC hours-minutes-seconds triples will be missing from the
   list of UTC names. In this case the progression will be:
 
      ... DECEMBER 31 23:59:57
      ... DECEMBER 31 23:59:58
      ... JANUARY   1 00:00:00
 
   Since 1972 when leap seconds and the UTC system were introduced, a
   negative leap second has not occurred.
 
 
The Leapseconds Kernel (LSK)
 
   The primary difficulty with UTC strings is that it is not possible to
   predict which atomic times will correspond to times during a UTC leap
   second. Thus algorithms for converting between UTC and time systems that
   simply use a continuous set of numeric markers require knowledge of the
   location of leap seconds in the list of names. This is the purpose of
   the LEAPSECONDS kernel supplied with the Toolkit. To convert between UTC
   times and any other system, you must first load the leapseconds kernel
   via a call to the routine furnsh_c.
 
   LSK files conform to a flexible format called ``NAIF text kernel''
   format. The SPICE file identification word provided by itself on the
   first line of an LSK file is ``KPL/LSK''. Both the NAIF text kernel
   format and SPICE file identification word are described in detail in the
   Kernel Required Reading document, kernel.req.
 
 
Ephemeris Time (ET)
--------------------------------------------------------
 
   Ephemeris time is the uniform time scale represented by the independent
   variable in the differential equations that describe the motions of the
   planets, sun and moon. There are two forms of ephemeris time:
   Barycentric Dynamical Time (TDB) and Terrestrial Dynamical Time (TDT).
   Although they represent different time systems, these time systems are
   closely related.
 
 
Barycentric Dynamical Time (TDB)
 
   Barycentric dynamical time is used when describing the motion of bodies
   with respect to the solar system barycenter.
 
 
Terrestrial Dynamical Time (TDT)
 
   Terrestrial dynamical time is used when describing motions of objects
   near the earth. As far as measurements have been able to detect, TDT and
   TAI change at the same rate. Thus the difference between TDT and TAI is
   a constant. It is defined to be 32.184 seconds. At the zero point of
   TAI, TDT has a value of 32.184.
 
 
The Relationship between TDT and TDB
 
   TDB is believed to be in agreement with the time that would be kept by
   an atomic clock located at the solar system barycenter. A comparison of
   the times kept by a clock at the solar system barycenter with a TDB
   clock on earth would reveal that the two clocks are in close agreement
   but that they run at different rates at different times of the year.
   This is due to relativistic effects.
 
   At some times in the year the TDT clock appears to run fast when
   compared to the TDB clock, at other times of the year it appears to run
   slow. Let TDB0 be some fixed epoch on the TDB clock and TDT0 be a fixed
   epoch on the TDT clock (TDB0 and TDT0 do not necessarily have to be the
   same epoch). Any epoch, EPOCH, can be represented in the following ways:
   as the number of seconds TDB(EPOCH), that have elapsed since TDB0 on the
   TDB clock; or as the number of seconds, TDT(EPOCH), that have elapsed
   since TDT0 on the TDT clock. If we plot the differences TDB(EPOCH) -
   TDT(EPOCH) against TDB(EPOCH) over all epochs, we will find that the
   graph is very close to a periodic function.
 
   In CSPICE the difference between TDT and TDB is computed as follows:
 
      [1]      TDB - TDT =  K * sin (E)
 
   where K is a constant, and E is the eccentric anomaly of the
   heliocentric orbit of the Earth-Moon barycenter. This difference, which
   ignores small-period fluctuations, is accurate to about 0.000030
   seconds. Thus to five decimal places the difference between TDT and TDB
   is a periodic function with magnitude approximately 0.001658 seconds and
   period equal to one sidereal year.
 
   The eccentric anomaly E is given by
 
      [2]     E = M + EB sin (M)
 
   where EB and M are the eccentricity and mean anomaly of the heliocentric
   orbit of the Earth-Moon barycenter. The mean anomaly is in turn given by
 
      [3]     M = M0 + M1*t
 
   where t is the epoch TDB expressed in barycentric dynamical seconds past
   the epoch of J2000.
 
   The values K, EB, M0, and M1 are retrieved from the kernel pool. These
   are part of the leapseconds kernel. They correspond to the ``kernel pool
   variables'' DELTET/K, DELTET/EB, and DELTET/M. The nominal values are:
 
      DELTET/K               =    1.657D-3
      DELTET/EB              =    1.671D-2
      DELTET/M               = (  6.239996D0   1.99096871D-7 )
 
 
In the Toolkit ET Means TDB
 
   When ephemeris time is called for by Toolkit routines, TDB is the
   implied time system. Software that converts between the various time
   systems described here use TDB whenever ephemeris time is called for. We
   call this time ET. (You can convert a UTC time string to TDT times, but
   you must make two subroutine calls instead of one.)
 
   Ephemeris time is given in terms of seconds past a reference epoch. The
   reference epoch used throughout the Toolkit is the epoch J2000 (roughly
   noon on January 1, 2000). Using the Toolkit software, you can find out
   how many seconds the J2000 epoch is from right now.
 
 
Naming the Seconds of Ephemeris Time
--------------------------------------------------------
 
   Although ephemeris time is a formal time, within the limits of
   measurements it coincides with atomic time. As such we should be able to
   relate it to the expressions of time that we use everyday.
 
   However, ephemeris time is described as a count of ephemeris seconds
   past the ephemeris reference epoch (J2000). For most of us the
   expression
 
      -312819349 seconds past the ephemeris epoch J2000
 
   bears little relationship to the time system we use to organize our
   lives. For this reason, it is common to give names to the various
   ephemeris seconds in a manner analogous to the UTC naming of the seconds
   of TAI---as a calendar date and time of day. The above string
   corresponds to
 
      "1990 FEB 1 21:44:11 (TDB)"
 
   There is an important distinction between the names given to ephemeris
   seconds and the names used by the UTC system. The names assigned to
   ephemeris times never have leap seconds. The `seconds' component of the
   name is restricted to and includes all values from 0 to 59.999... . Thus
   the time string above does not represent the same moment in time as does
   ``1990 FEB 1 21:44:11 (UTC)'' There are two reasons. First, ephemeris
   time is ahead of atomic time by 32.184 seconds. Second, when a leap
   second occurs UTC strings fit an extra name into the sequence of valid
   UTC names. Thus it appears that UTC names fall behind ET names by a
   second after each leapsecond. At the present time UTC time strings
   appear to be 62.184 seconds behind ET time strings. This appearance is
   due to the fact that the two naming conventions are not the same. They
   simply have a lot of names in common.
 
   It is both fortunate and unfortunate that there is a huge set of common
   names between calendar dates ET and calendar dates UTC. Since there are
   relatively few leapseconds, a time given by an ET name is always close
   to the time in the UTC system having the same name. Thus for planning
   observations, you can know what day the observation will take place,
   whether or not you are likely to need a coat and how to arrange your
   daily activities around the observation. But for precise work you must
   pay attention to the difference between the two times systems. If in
   planning the observation of a stellar occultation by an asteroid the
   difference between the two naming systems is neglected, it is likely
   that the observation will be missed.
 
   The routine str2et_c will convert an ephemeris calendar date to seconds
   past the ephemeris epoch J2000.
 
 
Some Consequences of Leapseconds
 
   There is no way of predicting when future leapseconds will occur.
   Normally you can predict whether there will be a leapsecond in the next
   few months, but beyond this predictions of leapseconds are not reliable.
   As a result we cannot say with certainty when a particular future UTC
   epoch will occur. For example, suppose you have a timer that you can set
   to ``beep'' after some number of seconds have passed. If this timer
   counts seconds perfectly without loosing or gaining time over decades,
   you cannot set it today to beep at midnight (00:00:00) January 1 (UTC)
   ten years from now---the number of leapseconds that will occur in the
   next ten years is not known. On the other hand, it is possible to set
   the timer so that it will beep at midnight January 1 (TDB). The TDB
   system does not have leapseconds. It is only necessary to know an
   algorithm (such as str2et_c) for converting calendar epochs TDB to
   seconds past some reference epoch in order to determine how to set the
   timer to beep at the correct epoch.
 
   Any given Leapseconds Kernel will eventually become obsolete. Sometime
   after the creation of any Leapseconds Kernel there will be new
   leapseconds. When future leapseconds occur the old Leapseconds Kernel
   will no longer correctly describe the relationship between UTC, TDT and
   TDB for epochs that follow the new leapsecond. However, for epochs prior
   to the new leapsecond, the old kernel will always correctly describe the
   relationship between UTC, TDT and TDB.
 
 
Computing UTC from TDB (deltet_c)
--------------------------------------------------------
 
   Below are a few epochs printed out in calendar format in both the TDT
   and UTC time systems.
 
      1996, Oct 11, 12:01:02.1840  (TDT)
      1996, Oct 11, 12:00:00.0000  (UTC)
 
      1996, Oct 12, 12:01:02.1840  (TDT)
      1996, Oct 12, 12:00:00.0000  (UTC)
 
      1996, Oct 13, 12:01:02.1840  (TDT)
      1996, Oct 13, 12:00:00.0000  (UTC)
 
      1996, Oct 14, 12:01:02.1840  (TDT)
      1996, Oct 14, 12:00:00.0000  (UTC)
 
      1996, Oct 15, 12:01:02.1840  (TDT)
      1996, Oct 15, 12:00:00.0000  (UTC)
 
   At least in October 1996, it's clear that if you have either TDT or UTC
   you can construct the corresponding representation for the same epoch in
   the UTC or TDT system by simply subtracting or adding 62.184 seconds.
 
   If you don't worry about what happens during a leapsecond you can
   express the above idea as:
 
      [4]           DeltaTDT =  TDT - UTC
 
   For all epochs except during UTC leapseconds the above expression makes
   sense. DeltaTDT is simply a step function increasing by one after each
   leapsecond. Thus DeltaTDT can be viewed as a step function of either UTC
   or TDT.
 
   If you rearrange this expression, you can get
 
      [5]           UTC = TDT - DeltaTDT
 
   Since, TDT can be expressed as seconds past J2000 (TDT), the above
   expression indicates the UTC can be expressed as some count of seconds.
   This representation is referred to by the dubious name of ``UTC seconds
   past J2000.'' If you write down the UTC calendar time string
   corresponding to an epoch and count the number of seconds between that
   calendar expression and the UTC calendar expression ``January 1, 2000
   12:00:00'' and ignore leapseconds, you get the value of UTC in the
   expression above.
 
   In practice this expression is broken down as follows:
 
      [6]           UTC  =  TDT - DeltaTA - DeltaAT
 
   where
 
                    DeltaTA =  (TDT - TAI)
 
   and
 
                    DeltaAT =  DeltaTDT - DeltaTA
 
   The value DeltaTA is a constant, its value is nominally 32.184 seconds.
   DeltaTA is a step function. These two variables appear in the
   leapseconds kernel.
 
   If we combine equation [6] above with equation [1] from the section
   ``The Relationship between TDT and TDB'' we get the following expression
 
      [7]           TDB - UTC =  DeltaTA + DeltaAT + K*sin(E)
 
   This last value is called DeltaET and is computed by the CSPICE routine
   deltet_c. The various values that are used in the computation of DeltaET
   are contained in the Leapseconds Kernel. Indeed, a Leapseconds Kernel
   consists of precisely the information needed to compute DeltaET. Below
   is a sample Leapseconds kernel.
 
      \begindata
 
      DELTET/DELTA_T_A       =   32.184
      DELTET/K               =    1.657D-3
      DELTET/EB              =    1.671D-2
      DELTET/M               = (  6.239996D0   1.99096871D-7 )
 
      DELTET/DELTA_AT        = ( 10,   @1972-JAN-1
                                 11,   @1972-JUL-1
                                 12,   @1973-JAN-1
                                 13,   @1974-JAN-1
                                 14,   @1975-JAN-1
                                 15,   @1976-JAN-1
                                 16,   @1977-JAN-1
                                 17,   @1978-JAN-1
                                 18,   @1979-JAN-1
                                 19,   @1980-JAN-1
                                 20,   @1981-JUL-1
                                 21,   @1982-JUL-1
                                 22,   @1983-JUL-1
                                 23,   @1985-JUL-1
                                 24,   @1988-JAN-1  )
 
      \begintext
      DELTET/DELTA_T_A  corresponds to DeltaTA in equation [7].
      DELTET/K          corresponds to K in equation [7].
      DELTET/EB         corresponds to EB in equation [2].
      DELTET/M          corresponds to M0 and M1 of equation [3].
      DELTET/DELTA_AT   corresponds to DeltaAT of equation [7].
                        Note that this expression gives the
                        points on the UTC scale at which
                        DeltaAT changes.
 
   Although NAIF recommends against it, you could modify this file to alter
   the conversion. For example, until 1985 JPL's Orbit Determination
   Program (ODP) set used a value of 32.1843817 for DeltaTA, and some older
   CRS tapes were created using this value in the conversion from TAI to
   TDT. The value returned by deltet_c can be made compatible with these
   tapes by replacing the current value (32.184, exactly) with the older
   value. Also, JPL'S Optical Navigation Program (ONP) set does not use the
   periodic term (K sin E) of the difference TDB-TDT. Setting the value of
   K to zero eliminates this term.
 
 
Problems With the Formulation of DeltaET
 
   As we pointed out above, the expression ( TDT - UTC ) is meaningful as
   long as you stay away from leapseconds. If you write down the TDT and
   UTC representations for an epoch that occurs during a leapsecond you
   will have something like this:
 
      1996 Jan 01, 00:01:01.6840  (TDT)
      1996 Dec 31, 23:59:60.5000  (UTC)
 
   Given these two epochs, it is no longer clear what we should assign to
   the value TDT - UTC. Thus although equation [7] above provides a simple
   expression for computing the ``difference between UTC and TDB'', the
   expression fails to tell us how to convert between TDB (or TDT) and UTC
   during leapseconds. For this reason the CSPICE system does not use
   DeltaET when converting between TDB (or TDT) and UTC. Instead, the table
   of offsets corresponding to DeltaAT in the leapseconds kernel is
   converted to an equivalent table as shown below.
 
      Day Number of 1971-DEC-31     TAI seconds past 2000 at
                                    beginning of 1971-DEC-31
 
      Day Number of 1972-JAN-01     TAI seconds past 2000 at
                                    beginning of 1972-JAN-01
 
      Day Number of 1972-JUN-30     TAI seconds past 2000 at
                                    beginning of 1972-JUN-30
 
      Day Number of 1972-JUL-01     TAI seconds past 2000 at
                                    beginning of 1972-JUL-01
 
      Day Number of 1972-DEC-31     TAI seconds past 2000 at
                                    beginning of 1972-DEC-31
 
      Day Number of 1973-JAN-01     TAI seconds past 2000 at
                                    beginning of 1973-JAN-01
 
      Day Number of 1973-DEC-31     TAI seconds past 2000 at
                                    beginning of 1973-DEC-31
                 .                          .
                 .                          .
                 .                          .
 
   where the day number associated with a particular calendar date is the
   integer number of days that have passed since Jan 01, 0001 A.D. (on the
   extended Gregorian Calendar).
 
   Given an epoch to be converted between UTC and some other time system
   (call this other system `S'), we decompose the conversion problem into
   two parts:
 
       1.   converting between UTC and TAI,
 
       2.   converting between TAI and system S.
 
   To convert between TAI and UTC, we examine the above table to determine
   whether or not the epoch in question falls on a day containing a
   leapsecond or during a day that is 86400 seconds in length. Once the
   length of the day associated with the epoch has been determined, the
   conversion from UTC to TAI (or from TAI to UTC) is straight forward.
   (See the routine ttrans_ for details.) Having settled the problem of
   converting between TAI and UTC, the conversion between TAI and system S
   is carried out using the analytic expressions (equations [1], [2] and
   [3]) given above.
 
 
Spacecraft Clock (SCLK)
--------------------------------------------------------
 
   Most spacecraft have an onboard clock. This clock controls the times at
   which various actions are performed by the spacecraft and its science
   instruments. Observations are usually tagged with the spacecraft clock
   time when the observations are taken.
 
   Each spacecraft clock can be constructed differently. For Galileo the
   CSPICE spacecraft clock times looks like
 
      p/rrrrrrrr:mm:t:e
 
      p - partition number
      r - rim counts
      m - minor frame
      t - real time interrupt
      e - mod eight count
 
   When asking for the matrix which describes the pointing for some
   structure or instrument used to perform an observation, you will usually
   request this information by supplying the spacecraft clock string that
   was used to tag the observation. This string must usually be related to
   UTC or ET. Consequently it is necessary to load a file of ``spacecraft
   clock coefficients'' that enables CSPICE software to transform the
   spacecraft clock string into one of the other time systems. This file of
   spacecraft clock coefficients is loaded with the routine furnsh_c.
 
   A more detailed discussion of Spacecraft Clock is contained in the
   Required Reading file sclk.req that is included with the CSPICE Toolkit.
 
 
Julian Date
--------------------------------------------------------
 
   The Julian date system is a numerical time system that allows you to
   easily compute the number of days between two epochs. NAIF recognizes
   two types of Julian dates. Julian Ephemeris Date (JED) and Julian Date
   UTC (JDUTC). As with calendar dates used for ephemeris time and calendar
   dates UTC, the distinction between the two systems is important. The
   names of the two systems overlap, but they correspond to different
   moments of time.
 
   Julian Ephemeris Date is computed directly from ET via the formula
 
      jed = j2000_c + et/spd_c();
 
   where J2000 is a constant function that returns the Julian Ephemeris
   Date of the reference epoch for ET, and SPD is a constant function that
   gives the number or seconds per day.
 
   Julian Date UTC has an integer value whenever the corresponding UTC time
   is noon.
 
   We recommend against using the JDUTC system as it provides no mechanism
   for talking about events that might occur during a leapsecond. All of
   the other time systems discussed can be used to refer to events
   occurring during a leap second.
 
 
The abbreviation JD
 
   Julian date is often abbreviated as ``JD.'' Unfortunately, the meaning
   of this string depends upon context. For example, the CSPICE routine
   utc2et_c treats the string ``2451821.1928 JD'' as Julian Date UTC. On
   the other hand, the CSPICE routine tparse_c treats the same string as
   Julian Date TDB. Consequently, for high accuracy work, you must be sure
   of the context when using strings labeled in this way. Unless context is
   clear, it's usually safer to label Julian Date strings with one of the
   unambiguous labels: JDUTC, JDTDB, or JDTDT.
 
 
Appendix B. Parsing Time Strings
===========================================================================
 
   This appendix gives a detailed account of how the function tpartv_
   parses time strings. tpartv_ is the ``foundation'' function relied upon
   by str2et_c, utc2et_c, tparse_c and tpictr_c to accomplish the task of
   analyzing and assigning meaning to the components of a time string.
 
   This appendix is not for everyone. Unless you need to understand in
   great detail how parsing of strings is performed, you can safely skip
   this appendix. The discussion below is quite technical and mirrors very
   closely the code in tpartv_ that handles the parsing of time strings.
 
 
An Outline of the Parser
--------------------------------------------------------
 
   The first step in processing a time string is to scan it from left to
   right identifying various substrings. If a substring is encountered that
   cannot be identified, attempts to further process the string are
   abandoned.
 
   Having identified the components in the string as integers, months,
   weekdays, time systems, etc. An internal representation of the string is
   constructed. This representation is simply a list of the identified
   substrings in the order they are encountered. Each item in the list is
   called a token.
 
   Working with the list of tokens, various rules are applied to remove
   some tokens and combine others into new tokens. The process of
   combination and removal of tokens continues until all tokens belong to a
   special set of ``meaningful'' tokens or until no further combinations
   and removals can be performed. If processing stops before all tokens are
   meaningful, a diagnostic message is created and the string is regarded
   as un-parsable. If all of the tokens are meaningful, a compatibility
   check is performed on the tokens to make sure that they unambiguously
   specify an epoch.
 
   Once it is clear that an unambiguous epoch has been specified, the
   substrings corresponding to the meaningful tokens are converted into
   numeric representations or are noted so that the time conversion
   software can properly interpret the numeric components.
 
   Almost all of the work of manipulating tokens is carried out by CSPICE
   private routines. These routines are not considered part of the CSPICE
   public interface. Feel free to read and copy these routines. However, we
   strongly recommend that you not call these routines in your own code
   since we do not guarantee backward compatibility of these routines in
   future releases of the Toolkit.
 
 
Tokenizing the Input String
--------------------------------------------------------
 
   The first step in parsing a time string is to decompose it into
   recognizable substring components. This decomposition is done as
   follows:
 
   Starting with the next unexamined character (on the first pass this is
   the first character in the string), scan from left to right looking for
   one of the following classes of substrings:
 
       1.   a maximal sequence of digits forming an unsigned integer.
 
       2.   a maximal sequence of space characters
 
       3.   a tab character
 
       4.   a weekday (or abbreviation of a weekday of at least 3 letters)
 
       5.   a month name (or abbreviation of a month name of at least 3
            letters)
 
       6.   a time zone ( standard U.S. abbreviations)
 
       7.   a positive UTC offset specifier ( `UTC+' )
 
       8.   a negative UTC offset specifier ( `UTC-' )
 
       9.   a time system (TDT, TDB, UTC)
 
      10.   an era specifier ( `A.D.', `B.C.', `AD', `BC' )
 
      11.   a 12-hour clock specifier ( `A.M.', `P.M.', `AM', `PM' )
 
      12.   a Julian date specifier ( `JD' )
 
      13.   a day of year specifier ( `::' or `//' )
 
      14.   a period `.'
 
      15.   a dash `-'
 
      16.   a slash `/'
 
      17.   a colon `:'
 
      18.   a left parenthesis `('
 
      19.   a right parenthesis `)'
 
      20.   a single quote character (')
 
   Once the next substring has been identified, its boundaries and
   classification are stored in the next available location in the buffer
   reserved for the tokenized representation of the time string.
 
   The steps above are then repeated until the entire substring has been
   tokenized or a failure to recognize some substring occurs. If a failure
   occurs the location in the string is noted and a diagnostic message is
   created indicating the failure in the attempt to parse the string.
 
   When the tokenization is finished, there will be a list of tokens from
   which a string can be constructed that lists the class of each token.
   Each class of token is represented by a single character. By placing
   these characters in a string a simple list of token classes is
   maintained. The characters used for the remainder of this discussion are
   listed below.
 
      Q  stands for the quote character
      [  stands for the left parenthesis character
      ]  stands for the right parenthesis character
      ,  stands for the comma character
      -  stands for the dash character
      .  stands for the decimal point character
      /  stands for the slash character
      :  stands for the colon character
      N  stands for one of the symbols A.M. or P.M.
      O  stands for the symbol UTC+
      Z  stands for a time zone such as PDT, PSD, CDT,
      b  stands for a block of white space (spaces or tabs)
      d  stands for the day of year marker (// or ::)
      e  stands for the era (B.C. or A.D.)
      j  stands for Julian date
      m  stands for a month
      o  stands for the symbol UTC-
      s  stands for a time system (UTC, TDT, TDB)
      t  stands the ISO date-T-time separator
      w  stands for the day of the week
      i  stands for a sequence of digits
 
   Thus the list of token classifications corresponding to
 
      "1995 Jan 12 12:28:28"
 
   will be
 
      "ibmbibi:i:i"
 
 
Combining and Removing Tokens
--------------------------------------------------------
 
   Once an internal tokenized representation of the time string has been
   created, the internal representation is manipulated so that the meaning
   of the tokens is gradually discovered.
 
   There are 3 basic operations that can be performed on the tokenized
   representation:
 
       1.   A token can be ``removed'' from the representation based on its
            classification. This removal can be wholesale as in ``remove
            all tokens corresponding to the blank character'', or it can be
            positional as in ``remove the last token classified as a
            blank.''
 
       2.   A sequence of tokens can be combined into a single new token
            with a potentially new classification. For example you might
            have a subsequence of token classifications such as `i.i' in
            the tokenized representation that corresponds to an unsigned
            integer, a period, and another unsigned integer. Under suitable
            circumstances this sequence `i.i' might be replaced by `n' (for
            number).
 
       3.   A single token can be reclassified. For example you might have
            a token whose classification is `i' for `unsigned integer' and
            have it reclassified as an hour `H'
 
 
Initial Token Processing
--------------------------------------------------------
 
   The first phase of processing the tokenized time discovers any UTC
   offsets in the input string, abbreviated months, decimal numbers, and
   removes white space. The process proceeds as follows:
 
       1.   Token sequences that represent UTC time offsets are combined to
            form a single token with a new classification. (The character
            used for this new kind of token is `Z'.)
 
       2.   Months or weekdays that are followed by a period are combined
            to form a single token (month or weekday respectively). The
            motivation for this combination is to allow abbreviations such
            as ``Jan.'' It also allows strings such as ``January.''
 
       3.   The right most sequence of tokens of the form ``i.i'',
            (integer-period-integer) or ``i.'' (integer-period) is combined
            to form a single token ``n'' (number). This combination is
            performed only once in the token resolution process.
 
       4.   All blanks (``b'') are removed from the tokenization.
 
 
Julian Dates
 
   The string is now examined to see if the Julian date specifier `JD' is
   present. If so the following operations are performed. If no Julian date
   specifier is present, the steps below are skipped and processing resumes
   under the section ``Calendar Dates.''
 
       1.   Any token sequence of the form `[s]' ( left parenthesis - time
            system - right parenthesis) is transformed to the sequence
            `*s*'. The `*' token is then removed. This leaves just the time
            system (TDT, TDB, or UTC) specification in the tokenization.
 
       --   Note: Whenever a character in the token classification is
            replaced by `*', the next step is to remove all tokens
            classified as `*' from the token list. In the remainder of the
            discussion, we will not add the sentence describing the removal
            of all asterisks. It will be implicit that the asterisk is
            always removed after it is placed in the token list.
 
       2.   If the token sequence `[j]' (left parenthesis - Julian date
            specifier - right parenthesis) is present, it is replaced by
            `*j*'
 
       3.   If no number token, `n', (see above) is present in the
            tokenization, the left most integer (`i') is reclassified as a
            number ( `n' ).
 
       4.   If the token sequence `-n' ( dash - number ) appears in the
            token list, it is combined and classified as a number (`n').
            This allows for the input of negative Julian dates.
 
       5.   The Julian date specifier `j' is noted and removed from the
            token list.
 
       6.   Any system token (`s') present in the token list is noted and
            removed.
 
       7.   The numeric components of the string are converted to double
            precision values and the token list is checked for unresolved
            tokens. (The only thing that should be in the token list at
            this point is a single numeric token.)
 
       8.   The parsing process halts. Either the string was successfully
            parsed and a double precision value for the Julian date has
            been constructed or there were unresolved tokens in the token
            list and a diagnostic message has been created.
 
 
Calendar Dates
 
   If the Julian date specifier was not present in the token list, we
   assume that the string and token list represents some calendar date
   format. One consequence of this assumption is that the dash `-' is now
   assumed to be just a punctuation mark and not part of some number. ISO
   formats are given first priority in the scheme of token resolution. Note
   that ISO formats do not allow the inclusion of time systems, time zones,
   eras, or 12-hour clocks.
 
   Any integer class tokens (`i') whose corresponding substrings represent
   integers greater than or equal to 1000 are reclassified as years (`Y').
 
 
ISO Formats
 
   If the ISO separator token `T' is present, the string is treated as an
   ISO format string. If the token list matches one of the token patterns
   in the left column it is transformed to the corresponding item in the
   right column by removing punctuation and making the indicated
   transformations.
 
         Y-i-iT ........ YmD
         Y-i-iTi ....... YmDH
         Y-i-iTi:i ..... YmDHM
         Y-i-iTi:i:i ... YmDHMS
         Y-i-iTi:i:n ... YmDHMS
         Y-i-iTi:n ..... YmDHM
         Y-i-iTn ....... YmDH
         Y-iT .......... Yy
         Y-iTi ......... YyH
         Y-iTi:i ....... YyHM
         Y-iTi:i:i ..... YyHMS
         Y-iTi:i:n ..... YyHMS
         Y-iTi:n ....... YyHM
         Y-iTn ......... YyH
         i-i-iT ........ YmD
         i-i-iTi ....... YmDH
         i-i-iTi:i ..... YmDHM
         i-i-iTi:i:i ... YmDHMS
         i-i-iTi:i:n ... YmDHMS
         i-i-iTi:n ..... YmDHM
         i-i-iTn ....... YmDH
         i-iT .......... Yy
         i-iTi ......... YyH
         i-iTi:i ....... YyHM
         i-iTi:i:i ..... YyHMS
         i-iTi:i:n ..... YyHMS
         i-iTi:n ....... YyHM
         i-iTn ......... YyH
 
                         Y  ---  Year
                         m  ---  Month
                         D  ---  Day of Month
                         y  ---  Day of Year
                         H  ---  Hour
                         M  ---  Minute
                         S  ---  Second
 
   If the token list contains the ISO separator (`T') but the list does not
   match one of the patters shown above, the input string is regarded as
   erroneous.
 
 
Other Calendar Formats
 
   If the ISO separator is not part of the token list, we next do what we
   can to recognize years and note the presence of modifiers (time zone
   specification, era, 12-hour clock etc.)
 
       1.   If a two digit integer is preceded by the quote character ('),
            the pair of tokens is combined to a single token and
            reclassified as a year.
 
       2.   The following token transformations are performed:
 
      "[e]"  ---> "*e*" (parenthesized era to era)
      "[w]"  ---> "*w*" (parenthesized weekday to weekday)
      "[N]"  ---> "*N*" (parenthesized AM/PM   to AM/PM)
      "[Z]"  ---> "*Z*" (parenthesized time zone to time zone)
      "[s]"  ---> "*s*" (parenthesized time system to time system)
      "ie",  ---> "Ye"  (integer-era  to Year-era)
 
       3.   Eras, weekdays, AM/PM, time zones, time systems are noted and
            removed from the token list.
 
       4.   The string is examined for redundant commas, dashes, slashes
            periods, etc. If any are found the string is regarded as
            erroneous.
 
 
Built in Representations
 
   Having processed the token list to this point, we check to see if what
   remains is one of those in a large set of immediately recognized token
   lists. The complete list is shown below. As in the case of ISO formats,
   the left item is the token list, the right item is the transformation
   after removing delimiters. Note that the letter `d' stands for a
   day-of-year delimiter ( `//' or `::' ).
 
      Y-i-it......... YmD             i/i/ii:i:n..... mDYHMS
      Y-i-iti........ YmDH            i/i/ii:n....... mDYHM
      Y-i-iti:i...... YmDHM           i/i/ii:n....... mDYHM
      Y-i-iti:i:i.... YmDHMS          i:i:ii-i-Y..... HMSmDY
      Y-i-iti:i:n.... YmDHMS          i:i:ii/i/Y..... HMSmDY
      Y-i-iti:n...... YmDHM           i:i:ii/i/i..... HMSmDY
      Y-i-itn........ YmDH            i:i:iimY....... HMSDmY
      Y-i/........... Yy              i:i:imiY....... HMSmDY
      Y-i/i:i........ YyHM            i:i:ni-i-Y..... HMSmDY
      Y-i/i:i:i...... YyHMS           i:i:ni/i/Y..... HMSmDY
      Y-i/i:i:n...... YyHMS           i:i:ni/i/i..... HMSmDY
      Y-i/i:n........ YyHM            i:i:nimY....... HMSDmY
      Y-id........... Yy              i:i:nmiY....... HMSmDY
      Y-idi:i........ YyHM            i:ii-i-Y....... HMmDY
      Y-idi:i:i...... YyHMS           i:ii/i/Y....... HMmDY
      Y-idi:i:n...... YyHMS           i:ii/i/i....... HMmDY
      Y-idi:n........ YyHM            i:iimY......... HMDmY
      Y-it........... Yy              i:imiY......... HMmDY
      Y-iti.......... YyH             i:ni-i-Y....... HMmDY
      Y-iti:i........ YyHM            i:ni/i/Y....... HMmDY
      Y-iti:i:i...... YyHMS           i:ni/i/i....... HMmDY
      Y-iti:i:n...... YyHMS           i:nimY......... HMDmY
      Y-iti:n........ YyHM            i:nmiY......... HMmDY
      Y-itn.......... YyH             iYd............ yY
      Yid............ Yy              iYdi:i......... yYHM
      Yidi:i......... YyHM            iYdi:i:i....... yYHMS
      Yidi:i:i....... YyHMS           iYdi:i:n....... yYHMS
      Yidi:i:n....... YyHMS           iYdi:n......... yYHM
      Yidi:n......... YyHM            iiY............ mDY
      Yii............ YmD             iiYi........... mDYH
      Yiii........... YmDH            iiYi:i......... mDYHM
      Yiii:i......... YmDHM           iiYi:i:i....... mDYHMS
      Yiii:i:i....... YmDHMS          iiYi:i:n....... mDYHMS
      Yiii:i:n....... YmDHMS          iiYi:n......... mDYHM
      Yiii:n......... YmDHM           iiYn........... mDYH
      Yiiii.......... YmDHM           iid............ Yy
      Yiiiii......... YmDHMS          iidi:i......... YyHM
      Yiiiin......... YmDHMS          iidi:i:i....... YyHMS
      Yiiin.......... YmDHM           iidi:i:n....... YyHMS
      Yiin........... YmDH            iidi:n......... YyHM
      Yim............ YDm             iim............ YDm
      Yimi........... YDmH            iimi........... YDmH
      Yimi:i......... YDmHM           iimi:i......... YDmHM
      Yimi:i:i....... YDmHMS          iimi:i:i....... YDmHMS
      Yimi:i:n....... YDmHMS          iimi:i:n....... YDmHMS
      Yimi:n......... YDmHM           iimi:n......... YDmHM
      Yimn........... YDmH            iimii.......... YDmHM
      Yin............ YmD             iimiii......... YDmHMS
      Ymi............ YmD             iimiin......... YDmHMS
      Ymii........... YmDH            iimin.......... YDmHM
      Ymii:i......... YmDHM           iimn........... YDmH
      Ymii:i:i....... YmDHMS          imY............ DmY
      Ymii:i:n....... YmDHMS          imYi........... DmYH
      Ymii:n......... YmDHM           imYi:i......... DmYHM
      Ymin........... YmDH            imYi:i:i....... DmYHMS
      Ymn............ YmD             imYi:i:n....... DmYHMS
      Ynm............ YDm             imYi:n......... DmYHM
      i-Y/........... yY              imYn........... DmYH
      i-Y/i:i........ yYHM            imi............ YmD
      i-Y/i:i:i...... yYHMS           imi:i:iY....... DmHMSY
      i-Y/i:i:n...... yYHMS           imi:i:nY....... DmHMSY
      i-Y/i:n........ yYHM            imi:iY......... DmHMY
      i-Yd........... yY              imi:nY......... DmHMY
      i-Ydi:i........ yYHM            imii........... YmDH
      i-Ydi:i:i...... yYHMS           imii:i......... YmDHM
      i-Ydi:i:n...... yYHMS           imii:i:i....... YmDHMS
      i-Ydi:n........ yYHM            imii:i:n....... YmDHMS
      i-i-Y.......... mDY             imii:n......... YmDHM
      i-i-Yi:i....... mDYHM           imiii.......... YmDHM
      i-i-Yi:i:i..... mDYHMS          imiiii......... YmDHMS
      i-i-Yi:i:n..... mDYHMS          imiiin......... YmDHMS
      i-i-Yi:n....... mDYHM           imiin.......... YmDHM
      i-i-it......... YmD             imin........... YmDH
      i-i-iti........ YmDH            imn............ YmD
      i-i-iti:i...... YmDHM           inY............ mDY
      i-i-iti:i:i.... YmDHMS          inm............ YDm
      i-i-iti:i:n.... YmDHMS          miY............ mDY
      i-i-iti:n...... YmDHM           miYi........... mDYH
      i-i-itn........ YmDH            miYi:i......... mDYHM
      i-i/i:i........ YyHM            miYi:i:i....... mDYHMS
      i-i/i:i:i...... YyHMS           miYi:i:n....... mDYHMS
      i-i/i:i:n...... YyHMS           miYi:n......... mDYHM
      i-i/i:n........ YyHM            miYn........... mDYH
      i-idi:i........ YyHM            mii............ mDY
      i-idi:i:i...... YyHMS           mii:i:iY....... mDHMSY
      i-idi:i:n...... YyHMS           mii:i:nY....... mDHMSY
      i-idi:n........ YyHM            mii:iY......... mDHMY
      i-it........... Yy              mii:nY......... mDHMY
      i-iti.......... YyH             miii........... mDYH
      i-iti:i........ YyHM            miii:i......... mDYHM
      i-iti:i:i...... YyHMS           miii:i:i....... mDYHMS
      i-iti:i:n...... YyHMS           miii:i:n....... mDYHMS
      i-iti:n........ YyHM            miii:n......... mDYHM
      i-itn.......... YyH             miiii.......... mDYHM
      i/i/Y.......... mDY             miiiii......... mDYHMS
      i/i/Y/i:n...... mDYHM           miiiin......... mDYHMS
      i/i/Yi:i....... mDYHM           miiin.......... mDYHM
      i/i/Yi:i:i..... mDYHMS          miin........... mDYH
      i/i/Yi:i:n..... mDYHMS          mnY............ mDY
      i/i/i.......... mDY             mni............ mDY
      i/i/ii:i....... mDYHM           nmY............ DmY
      i/i/ii:i:i..... mDYHMS
 
   If the token list agrees with one of the items in the above list, the
   double precision value corresponding to each token is computed and the
   parsing process halts with success.
 
 
Last Resort Production Rules
--------------------------------------------------------
 
   If the token list did not match one of the built-in patterns above,
   several checks are performed to see if there is redundant information in
   the token list (duplicate time systems, eras, etc.) If any such
   duplicate items are located, the input string is diagnosed as erroneous.
 
   Assuming that the error checks just discussed do not produce an error
   diagnosis, the string is processed according to the following rules:
 
       1.   Commas, dashes, and slashes are removed from the token list.
            The resulting token list is then compared once more against the
            list of token patterns above. If there is a successful match,
            the parsing process halts with success.
 
       2.   The following list of transformations are attempted in the
            order indicated.
 
      "i:i:i:n"  ---> "D*H*M*S" (days, hours, minutes, seconds)
      "i:i:i:i"  ---> "D*H*M*S" (days, hours, minutes, seconds)
      "i:i:n"    ---> "H*M*S"   (hours, minutes, seconds)
      "i:i:i"    ---> "H*M*S"   (hours, minutes, seconds)
      "i:n"      ---> "H*M"     (hours, minutes)
      "i:i"      ---> "H*M"     (hours, minutes)
 
       3.   All colons are removed from the token list.
 
       4.   The following list of transformations are attempted in the
            order indicated.
 
      "<miiH" ---> "mDY"  (month, day, year)
      "<mi"   ---> "mD"   (month, day)
      "Siim>" ---> "SYDm" (seconds, year, day, month)
      "im>"   ---> "Dm"   (day, month)
      "miY>"  ---> "mDY"  (month, day, year)
      "Ymi"   ---> "YmD"  (year, month, day)
      "Smi"   ---> "SmD"  (seconds, month, day)
      "Mmi"   ---> "MmD"  (minutes, month, day)
      "imY"   ---> "DmY"  (day, month, year)
      "imH"   ---> "DmH"  (day, month, hour)
      "Yid"   ---> "Yy*"  (year, day-of-year)
      "iYd"   ---> "yY*"  (day-of-year, year)
      "Ydi"   ---> "Y*y"  (year, day-of-year)
 
      The characters "<" and ">" mean that the transformation is
      performed only if the token list occurs at the beginning or
      end respectively of the token list.
 
       5.   The token list is now examined to determine whether any
            unresolved numeric tokens remain. If unresolved numeric tokens
            are present, the input string is diagnosed as erroneous. If no
            unresolved components remain, the token list is checked for
            consistency. For example there can be only one of each type of
            token, and there must be a sufficient number of tokens present
            to unambiguously determine the epoch.
 
 
Conclusion
--------------------------------------------------------
 
   As can be surmised from the preceding discussion, it is very difficult
   to give a complete list of all token patterns that might yield a parsed
   time string. Nevertheless, we feel that the approach taken and the
   transformations applied will yield correct and consistent
   interpretations of the many ways people choose to represent time.
 
 
Appendix C: Document Revision History
--------------------------------------------------------
 
 
September 9, 2015
 
   For Mice, added mention of Mice routines cspice_timdef_set and
   cspice_timdef_get.
 
   Corrected typo in the start date for the applicable period of the
   Gregorian calendar when the MIXED calendar option is selected. The date
   was changed to October 5, 1582.
 
   Corrected formatting of the Utility Routines section: the ASCII versions
   of this document for Mice and Icy displayed function names truncated to
   8 characters (the problem did not occur in the HTML versions).
 
 
April 9, 2009
 
   Adapted for Icy and Mice.
 
   Added a note about the SPICE file identification word for LSK files.
 
 
December 23, 2004
 
   Replaced ldpool_c and other lower level loader routines with furnsh_c
   throughout the document.
 
 
February 2, 2004
 
   Performed a spell-check on text.
 
 
18 November 1997 --- Ed Wright
 
   This edition of the TIME required reading is cast for the C version of
   the SPICELIB library, CSPICE.
 
 
CSPICE naming conventions
 
   The CSPICE library is an implementation of the FORTRAN SPICELIB library
   in C. CSPICE is composed of C routines translated to C from FORTRAN by
   f2c, and a set of wrapper functions which allow a more C native
   interface to the f2c'd routines.
 
            A routine name which ends in an underscore, ``_'', is an f2c
            translated routine (pckopn_).
 
            A routine name ending in and underscore c, ``_c'', is a wrapper
            routine (mxm_c). It is strongly suggested that the user calls a
            wrapper routine whenever available as opposed to the f2c
            translated counterpart.
 
            A routine name in all capital letters (SPKEZR) is a SPICELIB
            FORTRAN routine.
 
            Variables is routine calls which have the suffix ``_len'' are
            the lengths of particular strings in the argument list.
 
 
22 July 1997
 
   This edition of TIME Required Reading documents the routine et2lst_c.
   This routine allows user's to easily convert Ephemeris Time (Barycentric
   Dynamical Time) to the local solar time at a user specified longitude on
   the surface of an object.
 
   In addition to the new routine et2lst_c, we document a slight extension
   of the set of time strings that are recognized by the CSPICE time
   software. This extension is documented in Appendix B.
 
 
15 October 1996
 
   This edition of TIME Required Reading is a substantial revision to the
   previous edition; this reflects a major enhancement of the CSPICE time
   software. This version describes the new time related software that was
   included in version N0046 of CSPICE . We also draw distinctions between
   the various levels of time conversion software that are available to
   Toolkit users.
 
   The following routines are new as of version N0046 of SPICELIB.
 
       str2et_c      tsetyr_c      ttrans_      jul2gr_
       timout_c      timdef_c      tpartv_      gr2jul_
       tpictr_c      tchckd_       tcheck_      texpyr_
 
 
2012 JUN 14, EDW (JPL)
 
   Edit to leapseconds description of the UT1 to UTC deviation. The correct
   IERS value is 0.9 seconds, the previous version of the document quoted
   0.7 seconds.
 
 
2009 APR 09, BVS (JPL)
 
   Previous edits.
 
 
30 June 1994
 
   This version differs substantially from the previous version of 13 April
   1992. Much of the description of the time software has been redone and
   sections added to describe how to modify time string parsing behavior
   and the conversion between uniform time systems.
 
 
13 April 1992
 
   This version differs from the previous version of 10 April 1991 in that
   it discusses the new routine, unitim_c, for converting between additive
   numeric time systems.
 
