In past weeks I have told people on at least two occasions that 2000 was not a leap year. I thought that leap years were those divisible by 4, unless it is divisible by 400, then it is not. However, I was properly informed this evening that I had the rule backwards. A leap year is any year divisible by 4, unless it ends with a 00 (e.g. 1800, 1900), then it is not a leap year. But if the 00 year is divisible by 400, then it is a leap year. So once every 400 years, we have a 00 year that is a leap year, but rest are not. There is a pretty convincing page that explains this.