The intentional date bug
A primary goal of computer programming is to write bug-free code. But did you know that Excel programmers created an intentional bug?
It's true. According to Excel, the year 1900 is a leap year. So if you enter the following formula, Excel won't complain, even though 29 February, 1900 is not an actual date:
=DATE(1900,2,29)
The reason for this error is compatibility. In the early days of personal computing, Lotus 1-2-3 was the most popular software available. Lotus programmers made the leap year mistake, and Microsoft programmers reproduced it so they could use the same date serial number scheme as 1-2-3.
Therefore, the days of the week prior to 1 March, 1900 are incorrect (e.g., 28 February, 1900 is really a Wednesday, not a Tuesday as report by Excel). In actuality, this is not a big deal because Excel doesn't even support dates prior to 1 January, 1900 -- which itself is an oddity.
Microsoft claims, perhaps rightfully so, that fixing the bug would create many more additional problems.