Microsoft Excel's longstanding issue with treating the year 1900 as a leap year stems from Lotus 1-2-3 compatibility requirements. When Microsoft Multiplan and Excel launched, they adopted the same serial date system as Lotus to ensure seamless worksheet transfers. This historical decision means Excel still incorrectly calculates dates before March 1, 1900, though it handles other leap years correctly.

The WEEKDAY function returns wrong values for these pre-1900 dates, though this affects less than 0.1% of users. Correcting this would require massive data updates and break compatibility with older documents and systems using Excel's date serial numbers. The trade-off favors preserving existing workflows over technical precision for an edge case.