CDF_EPOCH_DIFF
--------------

The CDF_EPOCH_DIFF function computes the difference between the source epoch,
either a scalar or an array of epoch values (date and time), against a base
epoch, also a scalar or an array of values. The epoch can be one of the
CDF formats, CDF_EPOCH, CDF_EPOCH16 or CDF_TIME_TT2000. While data of type
CDF_EPOCH and CDF_EPOCH16 can be compared with each other, data in
CDF_TIME_TT2000 can only be compared with the data of the same type.
If the base epoch is a scalar, it is used to compute the differences with
all values in the source epoch. If the base epoch is an array, same element in
the source and base epoch array is compared. It returns a scalar value or an
array of values, depending on the source epoch, in milliseconds or microseconds
for CDF_EPOCH and CDF_EPOCH16 data types, or nanoseconds for CDF_TIME_TT2000 
data type.

Syntax 
   Result = CDF_EPOCH_DIFF(epoch1, epoch2 [,/MILLI_SECONDS | ,/MICRO_SECONDS]) 

Arguments 
   epoch1   A single or an array epoch value(s) returned from call to
            CDF_EPOCH,/compute, or CDF_EPOCH16,/compute for CDF_EPOCH or 
            CDF_EPOCH16 data type, or CDF_TT2000,/compute for CDF_TIME_TT2000 
            data type, CDF_VARGET, or CDF_VARGET1.

   epoch2   A single or an array epoch value(s) returned from call to
            CDF_EPOCH,/compute, or CDF_EPOCH16,/compute for CDF_EPOCH or 
            CDF_EPOCH16 data type, or CDF_TT2000,/compute for CDF_TIME_TT2000 
            data type, CDF_VARGET, or CDF_VARGET1.

Keywords 
   MILLI_SECONDS    The default for CDF_EPOCH and CDF_EPOCH16 data comparison 
                    and not applicable to CDF_TIME_TT2000 data type.
                    The resolution is in milliseconds. It indicates that
                    the sub-millisecond portions are ignored for CDF_EPOCH16
                    data type. It has no effect to CDF_EPOCH data type.
   MICRO_SECONDS    The resolution is in microseconds and the 
                    sub-microseconds portions of the data are ignored for
                    CDF_EPOCH16 data type. CDF_EPOCH data is expanded to 
                    microseconds, but is not applicable to CDF_TIME_TT2000.

Examples
   ; For CDF_EPOCH data type
   cdf_varget, id, "Epoch", ep, rec_count=1000,/zvariable
   cdf_epoch, base, 2005,6,1,10,18,17,2,/compute
   ret= cdf_epoch_diff(ep,base)
   help, ret
   ; For CDF_TIME_TT2000
   cdf_varget, id2, "Epoch", ep2, rec_count=1000,/zvariable
   cdf_tt2000, base2, 2005,6,1,10,18,17,2,3,4,/compute
   ret2= cdf_epoch_diff(ep2,base2)
   help, ret2

Version History 
  Introduced:  CDF V3.4.0