You can use the EDATE function to quickly add or subtract a specific number of whole months to or from a date.
The EDATE function requires two values (also referred to as arguments): the start date and the number of months that you want to add or subtract. To subtract months, enter a negative number as the second argument (for example, =EDATE("2/15/10",-5). This formula results in the date 9/15/09.