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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.]
From the wikipedia:
It's amazing that this level of detail is relevant to anything.
Without considering this, most people wouldn't be able to drive anywhere they haven't been before anymore.
"Wouldn't be able to" is a bit of a stretch, since Thomas Maps existed long before GPS. But it wouldn't be so easy as it is now.