Library /sys$common/syshlp/mailhelp.hlb DIRECTORY, Qualifiers, /BEFORE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /BEFORE=date

    Selects messages received before the specified date. If you do
    not specify a date, Mail displays a listing of all the mail
    messages received before the current day (today). You can also
    specify the keywords YESTERDAY and TODAY. For example, the
    command DIRECTORY/BEFORE=YESTERDAY would give you a listing of
    all mail messages received before yesterday.