VMS Help BACKUP Command, Qualifiers, /BACKUP  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Input File-Selection Qualifier

    Selects files according to the BACKUP date written in the file
    header record by the BACKUP/RECORD command. Use this qualifier
    with the /BEFORE or /SINCE qualifier, either of which provides
    the system time value BACKUP uses to determine whether to process
    a file.

    When you use /BACKUP with /BEFORE, files with a BACKUP date prior
    to the specified date or time are selected. Files with no BACKUP
    date (that is, /RECORD was not specified when the file was saved
    or copied) are also selected.

    When you use /BACKUP with /SINCE, files with a BACKUP date equal
    to or later than the specified date or time are selected. Files
    with no BACKUP date (that is, /RECORD was not specified when the
    file was saved or copied) are not selected.

    Use the /BACKUP file-selection qualifier only with Files-11
    Structure Levels 2 and 5 volumes. You cannot use /BACKUP with the
    /CREATED, /MODIFIED, or /EXPIRED qualifiers in an image operation
    or in a physical operation.