One of the more interesting sets of problems when writing Internet software is working with dates and times.
Pretty quickly, you learn to account for different time zones, the dateline and the mishmash of rules for daylight savings. Much less frequently, but more interestingly, differing calendars come into play.
In that vein, here are a couple of fun facts:
A European living in China in 1911 would have celebrated New Year's Eve on December 18th.
In 1918, in Estonia, Latvia and Lithuania, the month of February was fifteen days long.
In Sweden in 1712, February was 30 days long.
When George Washington was born, the date was February 11, 1731. It is now February 22, 1732.
When Alaska was purchased from Russia, the date for the US delegation was October 18th, 1867. The date for the Russian delegation was October 7th, 1867.
Needless to say, the transition from Julian to Gregorian calendars was a bit jarring - all the more so for being done over the course of almost 350 years.