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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.]
I wonder what fraction of xkcd readers have already seen this video... I imagine it is quite high. I know I've watched it multiple times. It's a good one.
Tom Scott and Veritaseum.