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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.]
From the wikipedia:
It's amazing that this level of detail is relevant to anything.
Without considering this, most people wouldn't be able to drive anywhere they haven't been before anymore.
"Wouldn't be able to" is a bit of a stretch, since Thomas Maps existed long before GPS. But it wouldn't be so easy as it is now.
Most people couldn't use a Thomas guide in the mid 90s either.
I was about when I navigated using an AAA TripTik to get my mom and younger brothers through an 1600 mile road trip. We also had AAA guidebooks for along the path when I had to help pick a motel along the way because a torrential rain slowed us down. It was a fun game of figuring out how far it was, whether it had enough stars (mom said at least 2, but the more the better), and the best price.
It was the late 80s though, and I didn’t have a game boy, so it was kinda the only form of entertainment.