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.]
Well in a very strict sense one can't really say "never" (unless you can see into the Future), but it's probably safe to go along with "It's highly unlikelly and if it does happen I'll fix it or will be long dead so won't care".