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

Procedure

      STR2ET ( String to ET )
 
      SUBROUTINE STR2ET ( STRING, 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

Declarations

 
      IMPLICIT NONE
      CHARACTER*(*)         STRING
      DOUBLE PRECISION      ET
 

Brief_I/O

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

Detailed_Input

     STRING     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 STRING are spelled
                out in great detail in the 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 STRING.

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) If a time zone is specified with hours or minutes components
        that are outside of the normal range, the error
        SPICE(TIMEZONEERROR) will be signaled.
        

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 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 68 ( 69 - 99 ) are regarded as being an
      abbreviation with the '19' suppressed (1969 - 1999).  Years
      smaller than 69 ( 00 - 68 ) are regarded as being an
      abbreviation with the '20' suppressed (2000 - 2068).

      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 1969 to 2068.

      You may modify these defaults through the routines TIMDEF
      and TSETYR (an entry point of TEXPYR).

      You may:

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

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

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

     See the routine TEXPYR and TIMDEF 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
     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 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 TIMDEF
         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
         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.  A complete discussion of
      permissible formats is given in the SPICE 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 SPICE routine TSETYR which is an entry point in the SPICE
      module TEXPYR.  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.  The small
      program below gives you a simple way to experiment with
      STR2ET.  (Note that erroneous inputs will be flagged by
      signaling an error.)

      To run this program you need to:

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

      PROGRAM

      CHARACTER*(64)        STRING
      CHARACTER*(64)        CALDR
      CHARACTER*(64)        DAYOFY
      CHARACTER*(127)       FILNAM

      DOUBLE PRECISION      ET


      First get the name of a leapseconds kernel, and load it.

      CALL PROMPT ( 'Leapseconds kernel: ', FILNAM )
      CALL FURNSH ( FILNAM )

      Leave some space on the screen and get the first trial string.
      If we get a blank input, we quit.

      WRITE (*,*)
      CALL PROMPT ( 'String to try: ', STRING )

      DO WHILE ( STRING .NE. ' ' )

         Convert the string to ET and then back to UTC calendar
         and day-of-year formats.

         CALL STR2ET ( STRING, ET )
         CALL ET2UTC ( ET, 'C', 0, CALDR  )
         CALL ET2UTC ( ET, 'D', 0, DAYOFY )

         Print the results.

         WRITE (*,*) 'Calendar    Format: ', CALDR
         WRITE (*,*) 'Day of year Format: ', DAYOFY

         Ask for another string and do it all again.

         WRITE (*,*)
         CALL PROMPT ( 'String to try: ', STRING )

      END DO
      END

Restrictions

     None.

Literature_References

     None.

Author_and_Institution

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

Version

    SPICELIB Version 1.3.1, 02-NOV-2009 (CHA)

        A few minor grammar errors were fixed in the header. 
        The header sections were reordered.

    SPICELIB Version 1.3.0, 31-AUG-2006 (NJB) (EDW)

        Bug fix:  routine formerly returned incorrect results
        in some cases on calls following calls for which a time
        zone was specified.

        Replaced reference to LDPOOL in header Examples section
        with reference to FURNSH.

    SPICELIB Version 1.2.2, 29-JUL-2003 (NJB)

        Various minor header corrections were made

    SPICELIB Version 1.2.1, 10-FEB-2003 (NJB)

        Corrected header typo.

    SPICELIB Version 1.2.0, 11-NOV-1997 (WLT)

        The previous versions of this routine did not correctly
        convert day-of-year strings in the TDB or TDT systems.
        They treated the day of year as year, month, day giving
        spectacularly wrong answers.

        In addition, comments concerning the default century for
        abbreviated years were updated to reflect changes to TEXPYR

    SPICELIB Version 1.1.0, 10-FEB-1997 (WLT)

        In the case that a time zone could not be parsed,
        this routine signaled an error and checked out without
        then returning.  This error has been corrected.

    SPICELIB Version 1.0.0, 15-NOV-1996 (WLT)
Wed Apr  5 17:47:32 2017