Microsoft KB Archive/181370

= Excel Incorrectly Assumes 1900 Is a Leap Year =

PSS ID Number: 181370

Article Last Modified on 8/19/2003

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The information in this article applies to:


 * Microsoft Excel 97 for Windows
 * Microsoft Excel for Windows 95 7.0
 * Microsoft Excel for Windows 5.0
 * Microsoft Excel 98 Macintosh Edition

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This article was previously published under Q181370



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 behaviors that may occur if this specific issue 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 actually 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 this behavior were to be corrected many problems would arise, including the following:
 * 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 might cause formulas in worksheets to work incorrectly.
 * Correcting this behavior would break serial date compatibility between Microsoft Excel and other programs that use dates.

If the behavior remains uncorrected, only one problem occurs:
 * 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 issue, please see the following article in the Microsoft Knowledge Base:

106339 : XL: Days of the Week Before March 1, 1900, Are Incorrect

NOTE: Microsoft Excel correctly handles all other leap years, including century years that are not leap years (for example, 2100). Only the year 1900 is incorrectly handled.

Additional query words: XL2 XL3 XL4 XL5 XL7 XL97 y2k year2000 year 2000 2,000 XL

Keywords: kbprb KB181370

Technology: kbExcel500 kbExcel95 kbExcel95Search kbExcel97Search kbExcel98 kbExcel98Search kbExcelMacsearch kbExcelSearch kbExcelWinSearch kbHWMAC kbOSMAC kbZNotKeyword3

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