/BACKUP=keyword(=value)

    This qualifier is used in combination with 3 possible
    keywords : BEFORE=date, SINCE=date or NONE. The backup
    date is used in for selecting files. Use SINCE and BEFORE or NONE,
    but not both. The NONE keyword returns files which don't have a
    backup date recorded. Examples :

    o  /BACKUP=BEFORE=date
    o  /BACKUP=(BEFORE=date,SINCE=anotherdate)
    o  /BACKUP=NONE