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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.]
With a 128 bit integer you can represent 340 undecillion (or sextillion if you use the long scale notation) seconds, which is equivalent to 10 nonillion (or quintillion, long scale) years. The universe will long have have stopped being able to support life by then because stars stopped forming (enough time would have passed it could have happened a hundred quadrillion (a hundred thousand billion, long form) times over assuming we start counting from the birth of the universe).