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Article ID: 214326 - Last Review: July 8, 2011 - Revision: 6.0

This article was previously published under Q214326

SYMPTOMS

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 assumption 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 would 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.
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.

REFERENCES

For additional information about this issue, click the article number below to view the article in the Microsoft Knowledge Base:
214058  (http://support.microsoft.com/kb/214058/EN-US/ ) XL2000: Days of the Week Before March 1, 1900 Are Incorrect
For additional information about determining whether a given year is a leap year, click the article number below to view the article in the Microsoft Knowledge Base:
214019  (http://support.microsoft.com/kb/214019/EN-US/ ) XL2000: Method to Determine Whether a Year Is a Leap Year

APPLIES TO
  • Microsoft Excel 98 for Macintosh
  • Microsoft Excel 2001 for Mac
  • Microsoft Excel 2004 for Mac
  • Microsoft Excel X for Mac
  • Microsoft Excel 97 Standard Edition
  • Microsoft Excel 2000 Standard Edition
  • Microsoft Excel 2002 Standard Edition
  • Microsoft Office Excel 2003
  • Microsoft Office Excel 2007
  • Microsoft Excel 2010
Keywords: 
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