Excel's default 1900 date system treats 1900 as a leap year, so it accepts the non-existent date 29 February 1900 and assigns it serial number 60. Every date from 1 March 1900 onward is therefore shifted one day higher than a correct serial-date scheme would produce. Excel inherited this from Lotus 1-2-3, whose leap-year test only checked divisibility by 4 and missed the Gregorian rule that century years must also divide by 400; Microsoft Multiplan and Excel copied it deliberately so worksheets could move between the programs without dates shifting.
In practice the offset cancels out for most arithmetic because both operands carry the same error, so the visible consequence is narrow: WEEKDAY returns wrong values for dates before 1 March 1900, and raw serial numbers exported to systems using correct date maths land one day off.
Steps to reproduce:
- Open Microsoft Excel with the default 1900 date system.
- Type
29/02/1900(or2/29/1900) into a cell. - Observe that Excel accepts it as a valid date rather than treating it as text — 1900 was not a leap year, so this date never existed.
- Format the cell as a number to see serial value 60, and enter
=WEEKDAY(DATE(1900,2,28))to see an incorrect weekday for a pre-March-1900 date.
Solution: None — Microsoft has documented this as won't-fix. Correcting it would shift almost every date in existing worksheets back by one day, change WEEKDAY results in live formulas, and break serial-date compatibility with other programs, which Microsoft judges worse than leaving the error in place.
Workaround: For dates before 1 March 1900, avoid WEEKDAY and compute weekdays with an external or hand-corrected method. When exporting raw serial numbers to systems using correct date arithmetic, subtract one day from any serial above 59. Excel's alternative 1904 date system starts at 1 January 1904 and sidesteps the phantom day entirely, though it offsets all serials by 1,462 days and mixing the two systems in one workbook introduces its own errors.
Severity: Minor (a deliberate, vendor-documented compatibility trade-off whose only real effect is wrong WEEKDAY values before March 1900 and one-day offsets in raw serial exchange — dates users actually work with are unaffected)
First seen: 1983 (originating in Lotus 1-2-3; inherited by Excel at its 1985 release)
Affected product: Microsoft Excel
Affected platforms: All platforms (Windows, macOS, web, and mobile — anywhere the 1900 date system is in use)
Affected versions: All versions using the 1900 date system, from the first Excel release through Microsoft 365 (also present in Microsoft Multiplan and Lotus 1-2-3)
Fixed versions: Not applicable — Microsoft states it will not be corrected
Links:
- https://learn.microsoft.com/en-US/office/troubleshoot/excel/1900-and-1904-date-system (retired)
- https://learn.microsoft.com/en-us/troubleshoot/microsoft-365-apps/excel/wrongly-assumes-1900-is-leap-year (updated 15 April 2026)
- https://www.makeuseof.com/microsoft-excel-has-been-carrying-the-same-date-bug-for-over-40-years/ (1 August 2026)