EOMONTH

Returns the date of the last day of a month which falls months away from the start date.

EOMONTH(StartDate; Months)

StartDate is a date (the starting point of the calculation).

Months is the number of months before (negative) or after (positive) the start date.

What is the last day of the month that falls 6 months after September 14 2001?

=EOMONTH(DATE(2001;9;14);6) returns the serial number 37346. Formatted as a date, this is 2002-03-31.

=EOMONTH("2001-09-14";6) works as well. If the date is given as string, it has to be in ISO format.