If you trade enough Imperial Bronze pieces to your Moogle, the maximum date of 23 January 2038 will be reached. If you trade any more pieces to it, the date will overflow and start back from 1970. As such, a friend's Mog Locker is currently valid until 1978.
Introducing a cap should fix that and if the new date is earlier than the old date, just set it to the cap and disable further trades. That, or switch to a 64bit number.
This could also be retroactively applied, so that if anyone's locker is earlier than the current date, set it to the cap.