GDATTIM

     GDATTIM is the date/time for the grid.

     Grids may contain two date/time fields, in which case the fields must 
     be separated using a colon.  If the grid to be selected contains only 
     one time, the colon and second time may be omitted.

     The standard format for a grid time is a character string

		YYMMDD/HHMMthhhmm
     where:
		YYMMDD	is the year, month, day
		/	is the date and time separator
		HHMM	is the hour, minute
		t	is the type
                            ( F=forecast  A=analysis  G=guess  I=initial )
		hhhmm	is the forecast hour, minute

     If t is blank, an analysis grid is assumed.  The hhhmm field cannot
     be omitted if the type is given.  If hhhmm has one, two, or three
     digits, they represent hours.  With four or more digits, zeros will
     be added at the beginning of the field.

     The field YYMMDD is called the date; HHMM is the time; t is the type; 
     hhhmm is the forecast time.  The fields may be abbreviated.  If the 
     input has no /, it is assumed to be the time part.  Any abbreviated 
     version of either date or time is assumed to be the part closest to 
     the /.  The rest of the date and time is obtained from the last time 
     in the file or data set.  If the type is missing, then A is assumed.

     The type and forecast time fields are used with forecast model data.
     If GDATTIM is 940831/0000F24, the grid to be found is the 24-hour
     forecast from the model run at 00Z on Aug 31.

     If a base time is specified in the first GDFILE entry, then the
     entry in GDATTIM should not contradict it.  If there is a
     contradiction, the GDATTIM entry overrides the entry in GDFILE.

     FIRST and LAST are valid values and have the following definitions.
     FIRST by itself means the first forecast time for the given cycle,
     when the times represent forecast periods. When the times are analyses,
     FIRST is the first time in the data set. Similar definitions apply
     for LAST.

     FIRST and LAST may be used in either the cycle or forecast parts of
     the date/time string. In general the format is cycleFfcst. This
     definition means that FIRSTFFIRST is a valid construct.
     
     A range of times, with an optional increment, may also be entered
     by the user.  Either the cycle portion of the time or the forecast
     portion may vary for the range.  However, both cannot vary for the
     same execution of the program.  Dual times may appear in a range
     and should always include an increment to avoid ambiguous situations.

     The value in GDATTIM may be overridden by specifying ^GDATTIM 
     with the grids to be found.  For example, 
     GFUNC = SUB (TMPF^28/12,TMPF^27/12) will compute the same 
     time difference as GFUNC = TDF (TMPF) with GDATTIM=28/12:27/12.
 
     Note that a grid from an objective analysis will have forecast
     type and time A00000, which is the default for a blank forecast
     type and time.