/sys$common/syshlp/helplib.hlb BACKUP Command, Qualifiers, /MODIFIED  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Input File-Selection Qualifier

    Selects files according to the value of the modified date field
    (the date the file was last modified) in each file header record.
    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.