How do you get the month number from a start date?

I need a formula to get the month number from the startdate. Not the numerical number of the month name, but the number of the month from the start date. So If I had a date that started on 2/14/2015. Month #1 would be 2/14/2015-3/14/2015. If anyone has the formula I would appreciate it

date ------------- month #
date ------------- month #
date ------------- month #
date ------------- month #
date ------------- month #

=DATEDIF(StartDate,EndDate,"M")