999
xkcd #2867: DateTime
(imgs.xkcd.com)
Alt text:
It's not just time zones and leap seconds. SI seconds on Earth are slower because of relativity, so there are time standards for space stuff (TCB, TGC) that use faster SI seconds than UTC/Unix time. T2 - T1 = [God doesn't know and the Devil isn't telling.]
Good point. You'd need at least 215 bits to represent all measurably distinct times (in multiples of the Planck time, approximately 10^-43 seconds) out to the projected heat death of the universe at 100 trillion (10^14) years. That should be sufficient for even the most detailed and lengthy simulation.