Try This
DateTime startDate = new DateTime(2013, 7, 9);
DateTime endDate = new DateTime(2013, 7, 9);
int months = endDate.Subtract(startDate).Days / 30;
or
((date1.Year- date2.Year)*12)+ date1.Month- date2.Month
or
date1.Subtract(date2).Days / (365.25 / 12)