When Microsoft Multiplan and Microsoft Excel were released, they also assumed that 1900 was a leap year. This assumption allowed Microsoft Multiplan and Microsoft Excel to use the same serial date system used by Lotus 1-2-3 and provide greater compatibility with Lotus 1-2-3. Treating 1900 as a leap year also made it easier for users to move worksheets from one program to the other.
Although it is technically possible to correct this behavior so that current versions of Microsoft Excel do not assume that 1900 is a leap year, the disadvantages of doing so outweigh the advantages.
If this behavior were to be corrected, many problems would arise, including the following:
- Almost all dates in current Microsoft Excel worksheets and other documents would be decreased by one day. Correcting this shift would take considerable time and effort, especially in formulas that use dates.
- Some functions, such as the WEEKDAY function, would return different values; this might cause formulas in worksheets to work incorrectly.
- Correcting this behavior would break serial date compatibility between Microsoft Excel and other programs that use dates.
- The WEEKDAY function returns incorrect values for dates before March 1, 1900. Because most users do not use dates before March 1, 1900, this problem is rare.