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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.]
It’s a programmer thing. As you’re typing the code, you may suddenly realize that the program needs to a assume certain things to work properly. You could assume that time runs at a normal rate as opposed to something completely wild when traveling close to the speed of light or when orbiting a black hole.
In order to keep the already way too messy code reasonably simple, you decide that the program assumes you’re on Earth. You leave a comment in the relevant part of the code saying that this part shouldn’t break as long as you’re not doing anything too extreme.