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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.]
Could you eli5 what frame and epoch are? I don't get why aren't unix timestamps an adequate way to store time, they seem pretty easy and intuitive