ForceLang, 123 bytes

    def s set
    s d datetime
    s z string.char 8
    s z z.repeat 16
    label 1
    io.write z
    io.write d.toTimeString
    d.wait 9
    goto 1

 `datetime.toTimeString` is backed in the reference implementation by Java's `DateFormat.getTimeInstance()`, which is locale-dependent, so you can set your default system locale to one that uses 24-hour time.