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

       /SINCE=date

    Selects all of the mail messages received on or after the
    specified date. The format for the date is dd-mmm-yyyy. If
    you do not specify a date, Mail selects all of the mail
    messages received on the current day. You can also specify the
    keywords YESTERDAY and TODAY. For example, the command READ
    /SINCE=YESTERDAY would select all mail messages received since
    yesterday.