A leap year, which occurs every four years, contains an additional day (February 29). Although the year 1900 was not a leap year, Excel treats it as such. In other words, when you type 2/29/1900 into a cell, Excel does not complain. It interprets this as a valid date and assigns a serial number of 60. If you type 2/29/1901, however, Excel correctly interprets it as a mistake and doesn’t convert it to a date. Rather, it simply makes the cell entry a text string. How can a product used daily by millions of people contain such an obvious bug?
The answer is historical. The original version of Lotus 1-2-3 contained a bug that caused it to consider 1900 as a leap year. When Excel was released some time later, the designers knew of this bug and chose to reproduce it in Excel to maintain compatibility with Lotus worksheet files.
Why does this bug still exist in later versions of Excel? Microsoft asserts that the disadvantages of correcting this bug outweigh the advantages. If the bug were eliminated, it would mess up hundreds of thousands of existing workbooks. In addition, correcting this problem would affect compatibility between Excel and other programs that use dates. As it stands, this bug really causes very few problems because most users do not use dates before March 1, 1900.