Working Out Number Of Accrual Months Between Dates

I need a simply excel formula or macro which calculates the numbers of months i need to accrue for certain costs. The data I have includes the start month of the invoice, the end month of the invoice (i.e. showing how long the invoice is relevant for, quite often 1 year), and obviously the current month.

(eg. a 12 month invoice with a start date of Oct 08, end date Sep 09, current May 09 = 8 mths to accrue)
(eg2. a 12 month invoice with a start date of Apr 09, end date of Mar 10, current May 09 = 2 mths to accrue)
(eg3. a 12 month invoice with a start date of Jun 09, end date of May 10, current May 09 = 0 mths to accrue)