I have a question reguarding Excel.

I have 2 colums on with the date a medical was completed and another the day
its due for review.

I need to add another colum that automaticaly updates days until it is due.
eg.
31/4/2006 31/4/2008 1 (Due in 1 Day)
Or
20/4/2006 20/4/2008 - 10 (Due 10days ago)

ECT ECT
Please email me if you need a better explanation