MacLochlainns Weblog

I fielded a question from blog reader on a three year old post about the EOMONTH (End Of Month) function in Excel. The reader want to write a mega-function (a term for putting two or more functions together to solve a problem, according to Mr. Excel). The function would return the first day of any month for the first day of the month and the first day of the next month for any day after the first day of the last month.

I wrote a quick little solution for the user, as shown below:

=IF(A1=EOMONTH(A1,-1)+1,EOMONTH(A1,-1)+1,EOMONTH(A1,0)+1)

The results would look like:

The user ran into a problem with the example because the date was being calculated and not an integer value. The formula returned the first day of the next month when it should have returned the first day of the current month. That meant the source value in the cell wasn’t an integer. It was a real number. Integer values in Excel are numeric values for 12:00:00 A.M. of any day, and real numbers can be any time of the day.

Unfortunately, the EOMONTH function is a pseudo-overloaded function, and the EOMONTH function takes an int or double (integer or real number) as it’s parameter. That’s because Excel only supports a NUMBER data type, which can be an integer or real number.

The EOMONTH function always returns an integer. The IF function I provided was designed to compare an integer-based date cell value against the result of the EOMONTH function. It wasn’t robust enough to support a comparison of 11/1/13 at 12:01 A.M. against the result of the EOMONTH function (11/1/13 at 12:00 A.M.). That’s why it returned the first of the next month instead of the first of the current month.

The more complete solution requires using the TRUNC function around the source date (B4 or C4 in the following example) to ensure the IF statement compares dates not time-stamps.

=IF(TRUNC(B4)=EOMONTH(B4,-1)+1,EOMONTH(B4,-1)+1,EOMONTH(B4,0)+1)

You should note that the TRUNC function effectively rounds down to the integer and removes the fractional portion of the real number before making a comparison. It’s an explicit casting process when we take proactive measures to ensure the value before the IF function compares it.