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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.]
Here's the shortlist of horrors I've had to deal with in my career:
Things I've read about but haven't needed to deal with personally:
The one thing I really don't care about is relativity
it's all relative anyway