Days of the week before March 1, 1900 are incorrect in Excel

When you use the WEEKDAY function to compute the day of the week for dates before March 1, 1900 in Microsoft Excel 2000, the function returns an incorrect result.

For example, if you use the WEEKDAY function to determine the day of the week for February 16, 1900, Microsoft Excel returns 5, which indicates that February 16 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.

This problem occurs when the following conditions are true:

• You use the WEEKDAY function to determine the day of the week for dates before March 1, 1900.
• You use the 1900 date system.

To work around this problem, add one to the result of the formula as in the following example:

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 February 29 ("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 you use the 1900 Date System in Microsoft Excel.

This problem does not occur if you are using the 1904 date system. When you use the 1904 date system and the WEEKDAY function to compute the day of the week for dates prior to January 1, 1904, the function returns a #VALUE! error.