If you can convert them both to DateTime objects, you can subtract one from the other to create a TimeSpan object. TimeSpan has method for returning the number of days it occupies.
That will give you total days.
Divide by 7 and take the integer portion, to get number of whole weeks. that also gives the number of sundays with an error of +/- 1.
OR:
Start at the first date and itterate up to the last date by adding a Day .AddDays(1) and checking to see if each day is a sunday, if not, itterate a number of days counter: NumDays++