Why are some countries a day behind/ahead others if time moves at the same rate everywhere?
In: Earth Science
Time moves at the same rate everywhere on Earth (barring things like relativity), but our concept of time isn’t based on an absolute truth, it’s based on things like “when the sun first comes up” or “when the sun is directly overhead in the sky” or “when it’s dark.
For most of human history, this wasn’t a huge deal, and we didn’t have a reason to go more formal than that. This slightly changed with the Industrial Revolution, as we now had artificial electric lights and factory work, but it was only local time that mattered for the most part, and if you traveled you would just readjust based on that, since traveling still took up a lot more time.
This started to change once we got trains, and then really needed a bit of formalization once flying across the oceans was common. It really hit a peak more recently with the internet and the ability to communicate near-instantly with any arbitrary part of the world at any time.
So, the concept of timezones was born somewhere in there. The idea being that each country would set its time according to when noon was, and that this would solve things. As part of this, we needed *somewhere* for the day to switch over, since if we didn’t we’d all be switching days at different times and it wouldn’t be super intuitive for anyone but the people who were in Greenwitch England (where the “standard time” is set for global timezones; you take GMT, Greenwitch Mean Time, and either add hours if you’re east of Greenwitch, or subtract hours if you’re west of Greenwitch), until you get to the International Date Line, which is (more or less) right on the other side of Greenwitch’s longitude line.
This is still an imperfect system, as [Tom Scott](https://www.youtube.com/watch?v=-5wpm-gesOY) will tell you, as there are all sorts of exceptions to the standard things. Part of it being that we’ve had to refine our concept of time as our scientific knowledge grew (for instance: we added in leap years because someone at some point realized that we kept having the season changes shift around, so that the start of each season would be a day later once every 4 years, and this is because the world goes around the sun not once every 365 days, but once every 365.25 days. And then we had to change it *again* because we’re actually something like 365.247 days per year, and the hack for this is that a year is a leap year if it’s divisible by 4 *and not* divisible by 100, but if it’s divisible by 100 *and* is divisible by 400, then it *is* a leap year again.)
And this is before you have to talk about leap seconds.
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