Microsoft Excel’s default 1900 Date System contains a deliberate date-calculation error inherited for compatibility with Lotus 1-2-3. Excel treats February 29, 1900, as a valid date even though 1900 was not a leap year. The defect is not caused by two-digit year storage: Excel represents dates with floating-point serial numbers, with 1.0 corresponding to January 1, 1900, in the 1900 system. That design choice allowed historical spreadsheets to remain interoperable, but it embedded a false calendar date into a widely used business tool.
The error also affects calculations for January 1 through February 28, 1900. Excel’s WEEKDAY function can return incorrect results during that interval, and dates earlier than January 1, 1900, cannot be displayed in the 1900 system, instead producing a #VALUE! error for negative serial values. Microsoft retained the behavior as a compatibility measure rather than correcting the underlying calendar logic, making it a lasting product limitation for archival, financial, scientific, and date-processing spreadsheets. The source identifies no recall, financial penalty, or individual executive accountability.