Returns the time difference between a given start and end time.
   The end time must be the same as or later than the start time.

   In newer versions of OpenVMS the lexical function F$DELTA_TIME
   does the same.

   TimeDiff can be used on any OpenVMS version and will return the
   time difference not only in OpenVMS format, but optionaly in
   seconds, milliseconds or minutes.

   Timediff will always create the following four symbols:

     TIMEDIFF$START   Start time given to timediff
     TIMEDIFF$END     End time given to timediff
     TIMEDIFF$FORMAT  Format used for output value
     TIMEDIFF$VALUE   Output value