The last *big* leap year bug i experienced was in 2000, because hardly anyone implements the full 4/100/400 leap year rule in their code. Modern libaries do (for the most part), but there's a lot of homebrew/DIY code from people who don't appreciate the subleties of date/time calculations.

Just for reference, the rules are:

1. A year is a leap year if it is divisible by 4,
2. unless it is divisible by 100, then it's not a leap year,
3. except when it's divisible by 400, then it IS a leap year.

At least these are the rules as of now (2024), this may or may not change in the next few centuries, depending on long-term IERS measurements. You know, the nice people with the nice atomic clocks that release the twice-yearly update to Bulletin C to say if we're going to have to deal with a leap second at the end of the period.

If you think leap years are a pain the buttocks, what do you think the average program does when a minute suddenly has 61 seconds...