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

       /BEFORE=date

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