XL: Microsoft Excel Incorrectly Assumes 1900 Is a Leap Year
ID: Q181370
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The information in this article applies to:
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Microsoft Excel for Windows, versions 2.x, 3.x, 4.x, 5.0, 5.0c
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Microsoft Excel for the Macintosh, versions 2.x, 3.x, 4.0, 5.0, 5.0a
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Microsoft Excel for Windows NT, version 5.0
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Microsoft Excel for Windows 95, versions 7.0, 7.0a
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Microsoft Excel 97 for Windows
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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 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 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.
If the problem 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 that 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.
For more information, please see the following article in the Microsoft
Knowledge Base:
Q118923 : XL: Method to Determine Whether a Year Is a Leap Year
For more information about how Microsoft products are affected by year
2000 (Y2K) issues, please see the following Microsoft World Wide Web site:
http://www.microsoft.com/technet/topics/year2k/default.htm
Additional query words:
XL2 XL3 XL4 XL5 XL7 XL97 y2k year2000 year 2000 2,000
Keywords : kbdta xlformat xlformula
Version : MACINTOSH:2.x,3.x,4.x,5.0,5.0a,98; WINDOWS:2.x,3.x,4.x,5.0,5.0c,7.0,7.0a,97
Platform : MACINTOSH WINDOWS
Issue type : kbprb
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