VMS Help DTM, DELETE, HISTORY, Command Qualifiers, /BEFORE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

   /BEFORE=time

   Deletes all history information from the history file dated prior
   to the specified date. The deleted information is replaced by a
   single entry stating that history information has been deleted
   from the history file.

   If you omit the /BEFORE qualifier, the default is to remove
   information that was logged prior to the time you enter the
   command. If you include the /BEFORE qualifier and do not specify a
   time, the default is TODAY.

   You can specify the time as an absolute, delta, or combination
   time value, or as one of the following keywords: TODAY, TOMORROW,
   or YESTERDAY. Digital Test Manager interprets TOMORROW as the time at
   which you enter the DELETE HISTORY command.