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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.]
Obligatory "Falsehoods programmers believe about time"
I really wish that list would include some explanations about why each line is a falsehood, and what's actually true. Particularly the line:
If the author has proof that some software will run on a space ship that is orbiting a black hole, I'd be really interested in seeing it.
Technically isn't the Earth itself a sort of space ship which is orbiting (...a star which is orbiting...) the black hole at the center of the Milky Way galaxy? Not really close enough for time dilation to be a factor, but still.
All links to the original article are dead and even archive.org doesn't have a capture either. I guess the argument is along the lines of "it might not be relevant, when you're scripting away some tasks for your small personal projects, but when you're working on a widely used library or tool - one day, it might end up on a space vessel to explore whatever."
E.g. my personal backup script? Unlikely. The Linux kernel? Somewhat plausible.
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.
Well in a very strict sense one can't really say "never" (unless you can see into the Future), but it's probably safe to go along with "It's highly unlikelly and if it does happen I'll fix it or will be long dead so won't care".