XL: Microsoft Excel Incorrectly Assumes 1900 Is a Leap Year

• Microsoft Excel for Windows, versions 2.x, 3.x, 4.x, 5.0, 5.0c
• Microsoft Excel for the Macintosh, versions 2.x, 3.x, 4.0, 5.0, 5.0a
• Microsoft Excel for Windows NT, version 5.0
• Microsoft Excel for Windows 95, versions 7.0, 7.0a
• Microsoft Excel 97 for Windows
• Microsoft Excel 98 Macintosh Edition

SUMMARY

Microsoft Excel incorrectly assumes that the year 1900 is a leap year. This article explains why the year 1900 is treated as a leap year and outlines the problems that may occur if this specific problem is corrected.

MORE INFORMATION

When Lotus 1-2-3 was first released, the program assumed that the year 1900 was a leap year even though it was not a leap year. This made it easier for the program to handle leap years and caused no harm to almost all date calculations in Lotus 1-2-3.

When Microsoft Multiplan and Microsoft Excel were released, they also assumed that 1900 was a leap year. This 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 the problem were to be corrected many problems would occur. For example, the following problems would occur:

• Almost all dates in current Microsoft Excel worksheets and other documents would be decreased by one day. Correcting this shift could take considerable time and effort, especially in formulas that use dates.
• Some functions, such as the WEEKDAY function, would return different values; this could cause formulas in worksheets to work incorrectly.
• Correcting this problem 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.

  For more information about this problem, please see the following article in the Microsoft Knowledge Base:

        ARTICLE-ID: Q106339
        TITLE     : XL: Days of the Week Before March 1, 1900, Are
                    Incorrect
  
  

For more information, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q118923
   TITLE     : XL: Method to Determine Whether a Year Is a Leap Year