Here's probably more than you ever wanted to know about the reasons behind the y2k bug and leap year calculations.
First thing to remember is that the vast majority (and I *do* mean vast) of software running business systems in the World today is written in COBOL - most COBOL storage is done with characters, not binary data (ie 2008 would be 4 bytes and human readable, not 2 and in binary).
The second thing to remember is that unlike today, storage was very expensive, so the less storage you use, the better ... and that's where the 2 digit year format came from, as an attempt to remove 2 bytes of storage by dropping the century from the year. That's where the original convention of using 2 digits came from.
The third thing to remember is that when a lot of these systems were written (from the 60's through to the 90's) no-one really believed that their code would still be running at the turn of the century, so didn't really worry about what would happen when the new century occurred - neither for the rolling over of the year back to zero, or the fact that it was a leap year.
The fourth thing to remember is that businesses don't like to spend money, so why replace an existing system when you can expand it or update it. Some of the software from the 60's is still running in some of our financial institutions, maybe not in it's original form, but the roots are still there.
All of those things together caused the Y2K scare. It was real too - the company I work for spent literally 100's of man-years ensuring that all the software we wrote could cope with the rollover, either by expanding the year to 4 digits, or by altering the software to cope correctly with 2 - I was paid very well for being on call over that new year and got no calls
The next big date problem will be in early 2038 when the 32 bit unix epoch date rolls over - unix records time as seconds passed since 01/01/1970, and the 32 bits used to record that will roll over at that time (it's a signed number, which is why the date of rollover isn't in the 2100's).
