One of the eternal complexities of computing is how to handle dates and times. Practice makes perfect, so goes the saying; a corollary to that saying is that if you only do something rarely, you don't get very good at it. Leap years are a good example, as they only occur once every four years (or so), which is not very often in the greater scheme of things, and means that they nearly always give rise to bugs.
Figuring out how to accurately handle dates and times in a computer is no easy task. People have been struggling with this problem for decades, and are continuing to struggle with it. For a recent example of some detailed and thorough thinking about dates and times, you might want to check out Jon Skeet's Noda Time, a .NET version of the well-known Joda Time library. Joda Time is rich and detailed and has lots of great documentation; Noda Time has some nice documentation, too, such as this page on Core Concepts and this page on choosing the correct datatype.
Of course, these are relatively recent libraries, and build on the work of others over decades. For a superb and encyclopedic treatment of dates and times in computing, you'll want to visit Paul Eggert's master page. It's an incredible resource; you could spend months just reading through all the reference material collected from that site. To cite just a few, you'll find a link to Gilbert Healton's lengthy article: The Best of Dates, The Worst Of Dates, as well as the National Institute of Standards and Technology's gorgeous history of timekeeping: A Walk through Time.
Eggert is an Internet legend for collecting, analyzing, cataloging, and providing all this information. But we nearly lost it all last fall when a lawsuit caused Eggert and his colleague Arthur David Olson to take down their web databases. Happily, the Electronic Frontier Foundation stepped in, and provided legal counsel, so access to these resources remains available.
I tend to think that this is one of those areas where Open Source makes a lot of sense. These are terribly detailed and intricate libraries, and it's really to everyone's advantage to have solid, reliable, accurate, and dependable date/time computations in every piece of software they use, so to me it makes perfect sense for us all to collaborate on a single library of shared software for these datatypes.
So if you might have felt tempted to scoff at this week's Internet outages, wondering how we computer programmers could mess up such a simple concept, rest assured that it's much more complicated than you might originally think, and consider becoming familiar with the extensive and excellent work that has already taken place in this area.