If you don’t know that leap years don’t always happen every 4 years, you really should read about the switch from the Julian to the Gregorian calendar. PostgreSQL’s documentation on the topic is also interesting, as well as the output of ncal -p.

On a related subject, the insertion of a leap second triggered a few interesting blog posts from Dave Jones (1, 2, 3), and also a Linux bug, reported on slashdot, and already fixed.

(Yes, that post was mostly an I-need-to-bookmark-this post.)