Note that datediff() will return a 1 month difference for:
datediff("m",date(2007,2,28),date(2007,3,1))
...even though there is only one day difference, since datediff just subtracts the months, regardless of the days. While that might work in your case, you should be aware of this. For more precision, you could use "d" and divide by 30.4375, which will be more accurate on average.
-LB