VMS Help CMS, INSERT, GENERATION, Qualifiers, /BEFORE=date-time  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Directs CMS to insert the element generation based on the
    /GENERATION qualifier or the latest generation on the main line
    of descent if there is no qualifier. The generation inserted is
    the latest ancestor of the generation determined above that
    pre-dates the specified time. If no generation pre-dates that
    time (i.e. generation 1 is later) then no generation is inserted.

    The time value can be an absolute, delta, or combination time
    value, or one of the following keywords: TODAY, TOMORROW, or
    YESTERDAY.  If the time value is a future value, CMS uses the
    current time.  If no time value is supplied, CMS used TODAY.