It's scary how much code out there is less than 4 years old, and even scarier how much of it is because someone decided to somehow rewrite it unnecessarily and thus introduce this bug. I don't even want to know how many people think the code they wrote will be in use for more than 4 years.
On the other end of the scale, the code base I worked on today has seen at least 7 separate leap days in production. Last week a manager asked if we expected any issues with the leap day, and we all just kind of laughed (and politely said no).
