EOMONTH Formula in Excel calculates the last date of a month from a start date. It can calculate dates occurring in the past or in the future. Returns the serial number for the last day of the month that is the indicated number of months before or after start_date. Use EOMONTH Formula to calculate maturity dates or due dates that fall on the last day of the month. EOMONTH Formula is good to work with dates in Excel. Microsoft Excel stores dates as sequential serial numbers so they can be used in calculations. By default, January 1, 1900 is serial number 1, and January 1, 2008 is serial number 39448 because it is 39,448 days after January 1, 1900. If start_date is not a valid date, EOMONTH returns the #NUM! error value. If start_date plus months yields an invalid date, EOMONTH returns the #NUM! error value.