No, let's say that your data starts January 1, 2011, and ends on March 17, 2011. We both know that this covers January, Febraury adn March, so it's three months.

So, if you had :

StartDate = DATE(2011,01,01)
EndDate = DATE(2010,03,17)
?MONTH(EndDate)-MONTH(StartDate)

it would only show a value of 2, meaning two months, but it actually covered a partial month for a 3rd month.