XL: EOMONTH Function May Be Incorrect If Date Is in February

ID: Q179545

The information in this article applies to:

Microsoft Excel 97 for Windows
Microsoft Excel 98 Macintosh Edition

SYMPTOMS

If you use the EOMONTH function to return the last day of a month, the function may return the first day of the next month instead.

For example, the EOMONTH function may incorrectly return 3/1/2100 (March 1, 2100) instead of 2/28/2100 (February 28, 2100).

CAUSE

This problem occurs when the date returned by the EOMONTH function meets the following conditions:

The date is in February.

The date is in a century year that is not evenly divisible by 400, for example:
2100, 2200, 2300, 2500, 2600, 2700, 2900, 3000

NOTE: This problem does not occur if the date is in year 1900 or 2000.

WORKAROUND

Because the EOMONTH function should always return a date at the end of the month, you can modify the formulas so that they subtract one from the date returned by the EOMONTH function if the date is not at the end of the month. This forces the function to return the correct date, for example:

A1: 1/1/2100
A2: =EOMONTH(A1,1)

The formula in cell A2 returns 3/1/2100, an incorrect date. However, if you change the formula

A1: 1/1/2100
A2: =IF(DAY(EOMONTH(A1,1))=1,EOMONTH(A1,1)-1,EOMONTH(A1,1))

the formula returns the correct date (2/28/2100) even though the date falls in February of a century year that is not evenly divisible by 400.

This is how the formula works:

If the day of the end of the month is 1, recalculate the end of the month, subtract one, and return the date. Otherwise, recalculate the end of the month and return the date.

STATUS

Microsoft has confirmed this to be a problem in the Microsoft products listed at the beginning of this article. This problem no longer occurs in Microsoft Excel 2000.

MORE INFORMATION

The EOMONTH function, which is included in the Analysis ToolPak, allows you to return the date of the last day in a month. For example, if you want to determine the last day of the month six months from today's date, use the following formula:

=EOMONTH(TODAY(),6)

If today is 1/13/98, the function returns 7/31/98: the last day of the month six months from today.

The problem described in this article occurs only when the EOMONTH function returns a date in February of a century year that is not a leap year, for example:

A1: 1/1/2000 B1: =EOMONTH(A1,1)
A2: 1/1/2100 B2: =EOMONTH(A2,1)
A3: 1/1/2200 B3: =EOMONTH(A3,1)
A4: 1/1/2300 B4: =EOMONTH(A4,1)
A5: 1/1/2400 B5: =EOMONTH(A5,1)

The formulas in cells B1 and B5 return the correct results (2/29/2000 and 2/29/2400) because the dates are in century years that are evenly divisible by 400.

The formulas in cells B2, B3, and B4 return incorrect results (3/1/2100, 3/1/2200, 3/1/2300) because the dates are in century years that are not evenly divisible by 400.

For more information about leap years, please see the following article in the Microsoft Knowledge Base:

Q118923 Method to Determine Whether a Year Is a Leap Year

Additional query words: XL97 y2k year2000

Keywords : kb2000 xladdins
Version : MACINTOSH:98; WINDOWS:97
Platform : MACINTOSH WINDOWS
Issue type : kbbug