September 12, 2026, 7:22 am | Read time: 3 minutes
Microsoft has maintained a known error in Excel for decades. The spreadsheet software incorrectly assumes that the year 1900 was a leap year. As a result, a February 29, 1900, appears in the date logic, even though this day never existed. The problem has been known since the software’s inception. Nevertheless, it persists in current versions. According to the company, correcting it would be more problematic than the error itself.
Why Excel Uses an Incorrect Date
For dates, Excel uses a system with sequential serial numbers. Each calendar day is assigned a number. January 1, 1900, has the serial number 1, January 2 has the number 2. This scheme continues for all subsequent dates.
The error occurs at the end of February 1900. In reality, the year was not a leap year, so February had 28 days. However, Excel includes February 29, 1900, and assigns it the serial number 60. As a result, March 1, 1900, receives the number 61. All subsequent date values are based on this numbering.
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Origin Lies in an Older Spreadsheet
The cause dates back to the 1980s. At that time, Lotus 1-2-3 was one of the most important spreadsheet programs. The same error already occurred there.
The background is the rule for leap years. Years divisible by four are generally considered leap years. However, century years have additional requirements. They must also be divisible by 400. Since this was not the case for 1900, it was not a leap year.
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Lotus 1-2-3 did not account for this exception and still treated the year as a leap year. As a result, February 29, 1900, was also introduced there. Microsoft later adopted the same date system to ensure compatibility with existing files.
Why Microsoft Doesn’t Remove the Error
According to Microsoft, the issue affects only the year 1900. Excel processes all other leap years correctly.
In a support article, the company states: “Although it is technically possible to correct this behavior so that current versions of Microsoft Excel do not assume 1900 is a leap year, the disadvantages of this approach outweigh the advantages.”
A change would directly impact the numbering system of date values. If March 1, 1900, were to receive the serial number 60 instead of 61, all subsequent dates would shift accordingly. This could affect existing spreadsheets, calculations, and formulas with date values. For this reason, Microsoft continues to accept the error, even though it has been known for decades.