Happy Leap day!
Today we celebrate keeping the astronomical year in line with the calendar year by inserting an extra day into our calendars. Now with modern clocks we go a bit further in our desire to try to line up the start of the calendar year with the astronomical year by adding in leap seconds as well.
Today is the 29th of February, the day where if you hard-coded that February always has 28 days or that the year is 365 days long your programs break (adding 365 days to get a 1 year time delta on the calendar comes to mind). Many programs make incorrect assumptions about the calendar that infrequent events such as this expose.
Here's the algorithm to determine if a year is a leap year:
if (year is not divisible by 4) then (it is a common year) else if (year is not divisible by 100) then (it is a leap year) else if (year is not divisible by 400) then (it is a common year) else (it is a leap year)
A lot of people get this calculation with dates semi-correct, which means that lots of edge case bugs will occur. Perhaps my favorite example of this is how Lotus 1-2-3 miscalculated 1900 to be a leap year and then Excel deliberately maintained bug-compatibility with the old Lotus bug and is itself also incorrect.
This is a good example of the complexities of handling calendar dates, it helps to use libraries, but keep in mind some of the libraries themselves also get this wrong too.