Your computer has to be able to calculate times for a variety of places over many years, in order to accurately determine time spans in different locations. Simple proposal, but actually doing that is just ghastly. (I've tried, and surrendered, twice. It's terribly hard.) Your OS has a series of rules that allow it to figure those things out, and you can actually read through them. The comments within that file are really fascinating, laying out the whole history of timezones, leap-seconds, and calendar changes. I'm an aficionado of good code comments, but this takes it to a whole 'nother level.