Alt text:
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 simpler and a lot easier for another engineer to look at and understand later, so they can verify that it's right or change it if it's wrong or we decide to do something a little bit different. it's also been reviewed and tested by a lot of people working in a lot of cases that are all a little bit different from one another, so the odds that their code is correct are better than the odds that my code is correct, all other things being equal