/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 SET FOLDER
    SINCE=YESTERDAY would select all mail messages received since
    yesterday.