EOMONTH (start_date,months)
EOMONTH returns the last day of the month which is months from the start_date.
If A1 contains 12/21/00 then EOMONTH(A1,0)=12/31/00, EOMONTH(A1,5)=5/31/01, and EOMONTH(A1,2)=2/28/01
MONTH.