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

• Microsoft Excel for Windows, versions 2.x, 3.0, 4.0, 4.0a, 5.0, 5.0c
• Microsoft Excel for Windows 95, version 7.0
• Microsoft Excel 97 for Windows
• Microsoft Excel for the Macintosh, versions 2.2, 3.0, 4.0, 5.0, 5.0a
• Microsoft Excel 98 for the Macintosh

SUMMARY

In Microsoft Excel, when you use the WEEKDAY function to compute the day of the week for dates prior to March 1, 1900, the function returns an incorrect result.

For example, if you use the WEEKDAY function to determine the day of the week of February 16, 1900, Microsoft Excel returns 5, which indicates that the 16th was a Thursday. The function should return 6, because February 16, 1900, was actually a Friday.

When you use the WEEKDAY function to find the day of the week for dates on or after March 1, 1900, the function returns the correct answer.

MORE INFORMATION

When the date system in Microsoft Excel was originally created, it was designed to be fully compatible with date systems used by other spreadsheet programs.

However, in this date system, the year 1900 is incorrectly interpreted as a leap year. Because there is no 29th of February ("leap day") in the year 1900, the day of the week for any date before March 1, 1900 (the day after the "leap day"), is not computed correctly.

Note that this only affects dates before March 1, 1900, and that the error only occurs when using the 1900 Date System in Microsoft Excel.