Just define new epoch at the turn of a century. Like a "UTD" or "UT2" that begins like 2200/01/01 00:00 that is actually UTC 2199/12/31 11:43:21 or something. Then hardcode the magic TAI number in the headers along the gravity constant and such. Solves the time_t problem too if done every centuries; 2^32 seconds = 136.102208 years. The biggest concern will be that kids at that time will start asking why there was never been UTB and UTA, which our oldest grandkids wouldn't actually know why.