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Procedure
Abstract
Required_Reading
Keywords
Brief_I/O
Detailed_Input
Detailed_Output
Parameters
Exceptions
Files
Particulars
Examples
Restrictions
Literature_References
Author_and_Institution
Version
Index_Entries

Procedure

   void str2et_c ( ConstSpiceChar * str,
                   SpiceDouble    * et   )

Abstract

   Convert a string representing an epoch to a double precision
   value representing the number of TDB seconds past the J2000
   epoch corresponding to the input epoch.

Required_Reading

   TIME

Keywords

   TIME


Brief_I/O

   VARIABLE  I/O  DESCRIPTION
   --------  ---  --------------------------------------------------
   str        I   A string representing an epoch.
   et         O   The equivalent value in seconds past J2000, TDB.

Detailed_Input

   str        is a string representing an epoch.  Virtually all common
              calendar representations are allowed. You may specify a
              time string belonging to any of the systems TDB, TDT,
              UTC.  Moreover, you may specify a time string relative to
              a specific UTC based time zone.

              The rules used in the parsing of `str' are spelled out in
              great detail in the CSPICE routine tpartv_. The basics
              are given in the Particulars section below.

Detailed_Output

   et        is the double precision number of TDB seconds past the
             J2000 epoch that corresponds to the input `str'.

Parameters

   None.

Exceptions

   1) The error SPICE(UNPARSEDTIME) is signaled if the
      string cannot be recognized as a legitimate time string.

   2) The error SPICE(TIMECONFLICT) is signaled if more than
      one time system is specified as part of the time string.

   3) The error SPICE(BADTIMESTRING) is signaled if any component
      of the time string is outside the normal range of usage.
      For example, the day January 35 is outside the normal range
      of days in January. The checks applied are spelled out in
      the routine tcheck_.

   4) The error SPICE(EMPTYSTRING) is signaled if the input
      string does not contain at least one character, since the
      input string cannot be converted to a Fortran-style string
      in this case.
      
   5) The error SPICE(NULLPOINTER) is signaled if the input string
      pointer is null.

Files

   None.

Particulars

   This routine computes the ephemeris epoch corresponding to an input
   string.  The ephemeris epoch is represented as seconds past the
   J2000 epoch in the time system known as Barycentric Dynamical Time
   (TDB).  This time system is also referred to as Ephemeris Time (ET)
   throughout the SPICE Toolkit.

   The variety of ways people have developed for representing times is
   enormous. It is unlikely that any single subroutine can accommodate
   the wide variety of custom time formats that have arisen in various
   computing contexts. However, we believe that this routine will
   correctly interpret most time formats used throughout the planetary
   science community. For example this routine supports ISO time
   formats and UNIX `date` output formats. One obvious omission from
   the strings recognized by this routine are strings of the form

        93234.1829  or 1993234.1829

   Some readers may recognize this as the epoch that is 0.1829
   days past the beginning of the 234'th day of 1993.  However,
   many other readers may regard this interpretation as a bit
   obscure.

   Below we outline some of the rules used in the interpretation
   of strings.  A more complete discussion of the interpretation
   of strings is given in the routine tpartv_.


   Default Behavior
   ----------------

   Consider 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
   ------

   If you add more information to the string, str2et_c can
   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

   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.

   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, 07:29:48 UTC

   The following U.S. time zones are recognized.

      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. This notation 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.  Thus 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)


   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.  Times in TDB are written as
 
     1988 June 13, 12:29:48 TDB

   TDT times are written as:

     1988 June 13, 12:29:48 TDT

   Finally, you may explicitly state that the time system is UTC

     1988 June 13, 12:29:48 UTC.


   Abbreviating Years
   ------------------

   Although it can lead to confusion, many people are in the
   habit of abbreviating years when they write them in dates.
   For example

      99 Jan 13,  12:28:24

   Upon seeing such a string, most of us would regard this
   as being 1999 January 13, 12:28:24 and not January 13 of
   the year 99.  This routine interprets years that are less
   than 100 as belonging either to the 1900's or 2000's.  Years
   greater than 49 ( 50 - 99 ) are regarded as being an
   abbreviation with the '19' suppressed (1950 - 1999).  Years
   smaller than 50 ( 00 - 49 ) are regarded as being an
   abbreviation with the '20' suppressed (2000 - 2049).

   Note that in general it is usually a good idea to write
   out the year.  Or if you'd like to save some typing
   abbreviate 1999 as '99.

   If you need to specify an epoch whose year
   is less than 1000, we recommend that you specify the era
   along with the year.  For example if you want to specify
   the year 13 A.D. write it as

     13 A.D. Jan 12

   When specifying the era it should immediately follow the year.
   Both the A.D. and B.C. eras are supported.


   Changing Default Behavior
   -------------------------

   As discussed above, if a string is unlabeled, it is regarded
   as representing a string in the UTC time system on the
   Gregorian calendar.  In addition abbreviated years are
   regarded as abbreviations of the years from 1950 to 2049.

   You may modify these defaults through the routines timdef_c_
   and tsetyr_c.

   You may:

     Set the calendar to be Gregorian, Julian or a mixture of
     two via the timdef_c;

     Set the time system to be UTC, TDB, TDT or any time zone
     via the routine timdef_c;

     Set the range of year abbreviations to be any 100 year
     interval via the routine tsetyr_c.

  See the routines texpyr_ and timdef_c for details on changing
  defaults.

  These alterations affect only the interpretation of unlabeled
  strings.  If an input string is labeled the specification
  in the label is used.

  If any component of a date or time is out of range, str2et_c
  regards the string as erroneous.  Below is a list of
  erroneous strings and why they are regarded as such.

     1997 Jan 32 12:29:29     --- there are only 31 days in January


     '98 Jan 12 13:29:29 A.M. --- Hours must be between 1 and 12
                                  inclusive when A.M. or P.M. is
                                  specified.

     1997 Feb 29, 12:29:20.0  --- February has only 29 days in
                                  1997. This would be ok if the
                                  year was 1996.


     1992 Mar 12 12:62:20     --- Minutes must be between 0 and 59
                                  inclusive.

     1993 Mar 18 15:29:60.5   --- Seconds is out of range for this
                                  date.  It would not be out of
                                  range for Dec 31 23:59:60.5 or
                                  Jun 30 23:59:60.5 because these
                                  can be leapseconds (UTC).

   Specifics On Interpretation of the Input String
   -----------------------------------------------

   The process of examining the string to determine its meaning is
   called "parsing" the string. The string is parsed by first
   determining its recognizable substrings (integers, punctuation
   marks, names of months, names of weekdays, 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, 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.

   To determine the meaning of the numeric tokens in the input string,
   a set of "production rules" and transformations 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.

   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.  str2et_c does not
       support such strings.

       No exponential components are allowed.  For example you
       can't specify the Julian date of J2000 as 2.451545E6.

   5)  The single colon (:) when used to separate numeric
       components of a string is interpreted as separating
       Hours, Minutes, and Seconds of time.

   6)  If a double slash (//) or double colon (::) follows
       a pair of integers, those integers are assumed  to
       represent the year and day of year.

   7)  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.

   8)  An integer followed" by "B.C." or "A.D." is regarded as
       a year in the era associated with that abbreviation.

   9)  All dates are regarded as belonging to the extended
       Gregorian Calendar (the Gregorian calendar is the calendar
       currently used by western society).  See the routine timedef_
       to modify this behavior.

   10) If the ISO date-time separator (T) is present in the string
       ISO allowed token patterns are examined for a match
       with the current token list.  If no match is found the
       search is abandoned and appropriate diagnostic messages
       are generated.

   11) 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, or day of year mark.  The day of year mark is a pair
       of forward slashes or a pair of colons.)

       Note the delimiters do not have to be the same. The pair
       of characters ",-" counts as two successive delimiters.

   12) White space and commas serve only to delimit tokens in the
       input string.  They do not affect the meaning of any
       of the tokens.

   13) If an integer is greater than 1000 (and the "JD" label
       is not present, the integer is regarded as a year.

   14) When the size of the integer components does not clearly
       specify a year the following patterns are assumed

       Calendar Format

           Year Month Day
           Month Day Year
           Year Day Month

           where Month is the name of a month, not its numeric
           value.

           When integer components are separated by slashes (/)
           as in 3/4/5.  Month, Day, Year is assumed (2005 March 4)


        Day of Year Format

           If a day of year marker (// or ::) is present, the
           pattern I-I// or I-I:: (where I stands for an integer)
           is interpreted as Year Day-of-Year.  However, I-I/ is
           regarded as ambiguous.

Examples

   Below is a sampling of some of the time formats that are acceptable
   as inputs to str2et_c.  A complete discussion of permissible formats
   is given in the CSPICE routine tpartv_ as well as the reference
   document time.req located in the "doc" directory of the Toolkit.

   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 > 68 and to treat it is 20xy otherwise. Thus '69 is
   interpreted as 1969 and '68 is treated as 2068. However, you may
   change the "split point" and centuries through use of the CSPICE
   routine tsetyr_c. See that routine for a discussion of how you may
   reset the split point.

   ** All epochs are regarded as belonging to the Gregorian calendar.
   We formally extend the Gregorian calendar backward and forward in
   time for all epochs.

   +  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.

 
   Suppose you would like to determine whether your favorite time
   representation is supported by str2et_c.  The small program below
   gives you a simple way to experiment with str2et_c.  (Note that
   erroneous inputs will be flagged by signaling an error.)

   To build and run this program you need to:

   1.  copy it to a file,
   2.  un-comment the obvious lines of code,
       and replace the default string with your test string
   3.  compile it,
   4.  link the resulting object file with CSPICE,
   5.  and place the leapseconds kernel in your current directory.

    #include <stdio.h>

    #include "SpiceUsr.h"

    char   *date = "Thu Mar 20 12:53:29 PST 1997";
    char   *leap = "naif0007.tls";

    main ()
       {

         furnsh_c ( leap      );
         str2et_c ( date, &et );

         printf ( "%f\n", et );
       }

Restrictions

   None.

Literature_References

   None.

Author_and_Institution

   C.H. Acton         (JPL)
   N.J. Bachman       (JPL)
   W.L. Taber         (JPL)

Version

   -CSPICE Version 1.1.5, 02-NOV-2009   (CHA)

      A few minor grammar fixes in the header.

   -CSPICE Version 1.1.4, 16-JAN-2008   (EDW)

      Corrected typos in header titles:
      
      Detailed Input to Detailed_Input
      Detailed Output to Detailed_Output
      
   -CSPICE Version 1.1.3, 12-NOV-2006 (EDW)

      Added Parameters section header.

   -CSPICE Version 1.1.2, 29-JUL-2003 (CHA) (NJB)

      Various minor header corrections were made.

   -CSPICE Version 1.1.1, 10-FEB-2002 (NJB)

      Corrected typo in header.

   -CSPICE Version 1.1.0, 08-FEB-1998 (NJB)

      Re-implemented routine without dynamically allocated, temporary 
      strings.  Exceptions section of header was updated.
 
   -CSPICE Version 1.0.0, 25-OCT-1997 (EDW)

Index_Entries

    Convert a string to TDB seconds past the J2000 epoch
Wed Apr  5 17:54:45 2017