EOMONTH /text/scalc/01/func_eomonth.xhp EOMONTH function

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 you specify the date directly, we recommend using the standard ISO 8601 format because this should be independent of your selected locale settings.
EOMONTH wiki page