Was the year 2000 a leap year?

Leap years, in which an extra day is inserted at the end of February, come every four years, right? Not so fast. The Gregorian calendar is a bit more complicated than that.

In order to qualify for the extra day, a year needs to be evenly divisible by four. But if it’s divisible by 100, it’s not a leap year. However, if you can divide it by 400, it is again. This is because the Earth’s rotation around the Sun is ever so slightly less than 365.25 days, and over the course of 400 years, you end up with three extra days if you have a leap year every four years. The year 2000, therefore, was a leap year, but 2010 wasn’t. And nor was 1900.

That is, unless you use Microsoft Excel. The venerable spreadsheet app incorrectly treats 1900 as a leap year, justifying it as a matter of backwards compatibility with the even more venerable Lotus 1-2-3, which contained the same error.

Fixes for this leap year problem tended to get bundled in with Y2K date fixes, but at other times, the incorrect calculation of leap years, or the failure to accept that they even exist, has caused issues. Witness the freezing of first-generation Zune 30 models on December 31, 2008, though they fixed themselves 24 hours later. In 1996, all 660 computers controlling two aluminum smelting plants in Australia and New Zealand shut down at midnight on December 31, as they couldn’t handle a year with 366 days. Sony’s PlayStation 3 consoles thought that 2010 was a leap year, which caused errors on March 1 that year. Gmail’s chat system, meanwhile, sent all chat records saved on February 29, 2012 back in time to December 31, 1969, and the same year, Microsoft Azure was taken offline on February 28 due to incorrect time calculations.