Set and retrieve the defaults associated with calendar
   input strings.

   None.

   VARIABLE  I/O  DESCRIPTION
   --------  ---  --------------------------------------------------
   action     I   is the kind of action to take "SET" or "GET".
   item       I   is the default item of interest.
   lenout     I   Length of list for output.
   value     I/O  is the value associated with the default item.

   action     is a word that specifies whether timdef_c sets the
              value associated with item or retrieves the value
              associated with item.  The allowed values for
              action are "SET" and "GET".  The routine is not
              sensitive to the case of the letters in action.

   item       is the default items whose value should be set or
              retrieved.  The items that may be requested are:

              item        Allowed Values
              ---------   --------------
              CALENDAR    GREGORIAN
                          JULIAN
                          MIXED

              SYSTEM      TDB
                          TDT
                          UTC

              ZONE        EST, EDT, CST, CDT, MST, MDT, PST, PDT
                          UTC+HR
                          UTC-HR       ( 0 <= HR < 13 )
                          UTC+HR:MN    ( 0 <= MN < 60 )
                          UTC-HR:MN

              The case of item is not significant.

   lenout     is the allowed length of the string when returning a
              value via a "GET".  The size described by lenout should
              be large enough to hold any possible output plus 1.

   value      if the action is "SET" then value is an input and
              is the value to be associated with item.  Note that
              value is checked to ensure it is within the range
              of allowed values for item.  If it is not within
              the expected range and appropriate error message
              is signalled.  The case of value is not significant.

   value      if the action is "GET" then value will be the
              value associated with the requested item.  Note that
              when time zones are set, they are translated to the
              UTC offset form ( UTC(+/-)HR[:MN] ).  When value is
              an output it will be in upper case.

   None.

   This routine exists to allow SPICE toolkit users to alter
   the default interpretation of time strings made by the
   routine str2et_c.

   Normally, unlabelled time strings are assumed to belong to
   the Gregorian Calendar and are UTC times.  However, you
   may alter the default behavior by calling timdef_c.

   Calendar
   --------

   You may set the calendar to be one of the following

   Gregorian   --- This is the calendar used daily the
                   Western Hemisphere.  Leap years occur in this
                   calendar every 4 years except on centuries
                   such as 1900 that are not divisible by 400.

   Julian      --- This is the calendar that was in use prior
                   to October 15, 1582.  Leap years occur every
                   4 years on the Julian Calendar (including all
                   centuries.)  October 5, 1582 on the Julian
                   calendar corresponds to October 15, 1582 of the
                   Gregorian Calendar.

   Mixed       --- This calendar uses the Julian calendar
                   for days prior to October 15, 1582 and
                   the Gregorian calendar for days on or after
                   October 15, 1582.

   To set the default calendar, select on of the above for value
   and make the following call.

      timdef_c ( "SET", "CALENDAR", lenout, value );


   System
   -------

   You may set the system used for keeping time to be UTC (default)
   TDB (barycentric dynamical time) or TDT (terrestrial dynamical
   time).  Both TDB and TDT have no leapseconds.  As such the time
   elapsed between any two epochs on these calendars does not depend
   upon when leapseconds occur.

   To set the default time system, select TDT, TDB or UTC for value
   and make the following call.

      timdef_c ( "SET", "SYSTEM", lenout, value );

   Note that such a call has the side effect of setting the value
   associated with ZONE to a blank.

   Zone
   -----

   You may alter the UTC system by specifying a time zone (UTC
   offset).  For example you may specify that epochs are referred
   to Pacific Standard Time (PST --- UTC-7).  The standard
   abbreviations for U.S. time zones are recognized:

      EST   UTC-5
      EDT   UTC-4
      CST   UTC-6
      CDT   UTC-5
      MST   UTC-7
      MDT   UTC-6
      PST   UTC-8
      PDT   UTC-7

   In addition you may specify any commercial time zone by using
   "offset" notation.  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 (based
   on the sign that follows "UTC") to get the
   local time zone.  Thus to specify the time zone of Calcutta you
   would specify the time zone to be UTC+5:30.  To specify the
   time zone of Newfoundland use the time zone UTC-3:30.

   To set a default time zone, select one of the "built-in" U.S.
   zones or construct an offset as discussed above.  Then make the
   call

      timdef_c ( "SET", "ZONE", lenout, value );

   If you "GET" a "ZONE" it will either be blank, or have the
   form "UTC+/-HR[:MN]"

   Note that such a call has the side effect of setting the value
   associated with SYSTEM to a blank.

   Suppose you wish to modify the behavior of str2et_c so that
   it interprets unlabeled time strings as being times in
   Pacific Daylight Time and that you want the calendar to use
   to be the "Mixed" calendar.  The following two calls will
   make the desired changes to the behavior of str2et_c

       timdef_c ( "SET", "CALENDAR", lenout, "MIXED" );
       timdef_c ( "SET", "ZONE"    , lenout, "PDT"   );

   None.

   1) If the action specified is not SET or GET the error
      SPICE(BADACTION) is signalled.

   2) If the item specified is not one the recognized items
      the error SPICE(BADTIMEITEM) is signalled.

   3) If the value associated with a "SET", item input
      is not one of the recognized items, the error
      SPICE(BADDEFAULTVALUE) is signalled.

   None.

   W.L. Taber      (JPL)
   E.D. Wright     (JPL)

   None.

   -CSPICE Version 1.0.1, 13-APR-2000 (NJB) 

      Made some minor updates and corrections in the header comments.

   -CSPICE Version 1.0.0, 4-FEB-1998   (EDW)

   Change time software defaults.
   Time Zones
   Gregorian and Julian Calendars