And it doesn't even mention all the bizarre things that have been done (for reasons good and bad) to time by various governments. Like adjusting from local solar time to standard GMT offset timezones (which involves skipping a given number of minutes and seconds or having them twice). Or introducing/abolishing/moving around daylight savings time. Or "super daylight savings time" with a 2 hour offset. Or moving from one side of the international date line to the other. And of course the real biggie: the Gregorian calendar reform that various countries adopted at different times between 1582 and the 1920s, skipping between 10 and 13 days depending on when they adopted it.
Oh man, I read that article years ago, lost the link and have been unable to find it since. Thanks! Any technical document which starts with "The measurement of time has a very long history, dating back to the first records of human civilization" and has a section titled "Political Time" gets a special place in my heart.
Also, computer systems have different Gregorian calendars. Your system could use the proper Gregorian calendar and a system you're communicating with could be using the proleptic Gregorian calendar. Most people don't notice the difference because dates aren't frequently slung around < 1582 in the modern world, but if a system has an idea of an "unset" date being equal to 1/1/1 it could lead to an error if transmitted to the other, resulting in an invalid date 12/30/0.
Not to mention dealing with the numerous differences in when the Gregorian calendar was adopted, which is great fun if you need to deal with dates even back to the early 1900's across borders.
E.g. the "October revolution" falls in November - Russia didn't change until the Bolcheviks took power in 1917. And Greece didn't switch until 1923... China switched in 1912, but different factions in the civil war used different systems and it wasn't until 1929 they got a single (Gregorian) calendar again.. And there are many other countries that switched "recently".
My favorite is when Daylight Savings Time started after an election in Brazil, but before the date for the election runoffs. Turned out the voting machines couldn't be changed to handle the time change. Solution? They just pushed back the date when DST started until after the runoffs.
http://statoids.com/tbr.html -- "Note that the government frequently changes its mind [about DST] at the last minute."
Let's keep some perspective. The vast majority of software applications don't use dates from the 18th century so it's fine to for your app to assume that the Gregorian calendar always was and always will be.
...except that there were countries that weren't using the Gregorian calendar until well into the 20th Century, so you can't even reliably deal with dates as recent as the mid-1900s across borders under that assumption.
Imagine a historical database of Russian birthdays, copied from historical archives by data-entry clerks who had no idea when Russia switched from the Julian to the Gregorian calendar. (For extra fun, imagine that half of the clerks doing the data entry converted the dates before entering them, and half didn’t.)
Software makes assumptions, though most decent time libraries will correctly combine local and date to show the skipped days. It might be a bit lazy though, running "cal sept 1752" will show the same for all locals as far as I can tell from the man page.
