content: Every so often, the year isn't a perfect 31,536,000 seconds(excluding leap years). So, rather than ignore the error like normal people, the anal people responsible for keeping "official" time add "leap seconds" to keep "official" time in line with "real" time. This is an extreme corner case that most people wouldn't think of when designing systems that invoke time(assuming they even know about them); needless to say, some code is going to be thrown for a loop when that extra second hits(especially if they happen to look at the clock right then).