Reddit reviews Calendrical Calculations

Ask what day, hour, minute, and second it will be in 30 days. Let's try to figure this out.

Some months have 30 days, others have 31, except for one fucker that has 28 days, except every 4 years it has 29, except it's still 28 if it's also a year evenly divisible by 100, except it still is if it's evenly divisible by 400. Write in all those exceptions.

Since we want to be accurate to the second, we need to take leap seconds into account. These aren't on a set schedule like leap years are. The International Earth Rotation and Reference Systems Service keeps track of how atomic clocks are tracking with the physical location of the Earth. They announce the need for a leap second every six months. You'll need to keep a database up to date with future leap seconds.

Need to cross a time zone? Now you've done it. You might think going from Pacific to Mountain timezones in the US just means adding or subtracting an hour. Except if you're going through Arizona; they don't believe in Daylight Savings Time in Arizona (good for them), so things depend on the time of year.

If you need to calculate sufficiently far back, then you need to account for the fact that there are 10 days missing in October of 1582.

If you happen to be be writing software for a few of the Old Calendarist sects of the Orthodox Church, you better not use any of that heritical Gregorian Calendar shit that puts Easter on the wrong day.

This is by no means comprehensive. There's literally a 500 page book on the subject of calculating dates. Modern timekeeping is the result of centuries of hacks upon hacks. By the time computers came along, it was already a huge mess with no way to clean it up.

Calendrical Calculations

It looks like I remembered it a little wrong. It deals with more than just western time. However, it is a bit incomplete. It doesn't talk as much about time of day as it does dates.

Either way, it's good enough to scare people away from re-writing date math libraries in 2 hours.

While you mention historical, the real fun begins when you need to check when a given location switched from one calendar to another and possibly also how that calendar works. If you need to do any of that for some software project I can recommend this excellent book on the topic by the guys who implemented it all in Emacs.

Going to mention one of my favorite programming books. Calindrical Calculations is a must read for anyone who wants to work with time in their code