How many days including 1/1/2008 through 1/31/2008.

If you use the between dates calc cell#-cell# you will get 30 when there are
actually 31 days including the 1st day.

The problem is when a payment is made for a period calculating the number of
days, including the date of receipt. e.g 4/15/2007 thru 5/14/2007 or
2/1/2007 thru 3/15/2007