I was glad to see that this included my favorite calendrical fact: the existence of February 30th, in just one year in Sweden.<p>It misses another good one, which is what happened in Alaska. Alaska's transition from Julian to Gregorian happened when ownership of the territory transferred from Russia to the United States. This also involved moving Alaska to the other side of the International Date Line (or more correctly, moving the Date Line to the other side of Alaska). The net effect was that, in Alaska, Friday, October 6th, 1867 was followed by Friday, October 18th.
If you ever read Umberto Eco's novel "Foucault's Pendulum" this is a key plot point. Specifically some shadowy "conspiracy" group was set to meet, but the meeting never happened because some countries had revised their calendar and some hadn't so they missed each other. Its a great novel about secret societies, conspiracy theories, and peoples willingness to believe them.
Fun fact: astronomers (and Nasa) use the Julian calendar for events (for example eclipses) before october 1582. You'd think they use UT, but they don't, so you can match historical dates with celestial events. Also worth noting, astronomical year numbering has a year 0, whereas BC/AD system does not, so that year 0 is 1 BC.<p>You have to make time corrections when you program something related to astronomy. In most things we program, UT is an absolute on which we can rely, but when we go very far away in the past it's not that clear anymore.
Unix seems to handle it well:<p>$cal 9 1752<p><pre><code> September 1752
Su Mo Tu We Th Fr Sa
1 2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
</code></pre>
there is a Bug on the man page, which has more to do with "what country are we in"
BUGS
The assignment of Julian--Gregorian switching dates to country codes is historically naive for many countries.
The Serbian Orthodox Church <i>still</i> hasn't switched from the Julian calendar. So now their religious holidays like Christmas are a couple weeks off from the rest of the world and will continue diverging.
That's cool, I hadn't ever heard about Sweden's double-leap years.<p>I did some research on calendars a while back, and notably, the conversion to Gregorian had a lot of religious overtones. Here's what I wrote about it:<p><a href="https://webcache.googleusercontent.com/search?q=cache:xrRtualQoz8J:https://curtisautery.appspot.com/5779342353235968" rel="nofollow">https://webcache.googleusercontent.com/search?q=cache:xrRtua...</a><p>(edit ...and Google cache is being ornery right now.)