Fill in the Before field with a time value indicating the
  time from which CMS should delete all previous history
  information.

  Entries are made in the history file specifying
  that a section of the history data has been removed. These
  entries are made at the locations in the history file where
  the lines were deleted.

  The time value can be an absolute, delta, or a combination
  time value, or one of the following keywords: TODAY,
  TOMORROW, or YESTERDAY.