Calculating Overdue Days

I have table with three columns:     1) TodaysDate,
           
           
           
                2)
DueBackDate,    
           
           
           
            3) DaysOverdue

The problem I am having is creating a formula in MS SQL, so that when
the 'DueBackDate' column is more than todays date, I want the
'DaysOverdue' column to be incremented accordingly.

I have tried creating an SQL Query something like this:

UPDATE rental

SET  DaysOverdue = DaysOverdue + 1

WHERE  ({ fn NOW() } > DueBackDate)

But the problem with this that it only increments the 'DaysOverdue'
field by 1. When I want it to basically subtract todays date by
'DueBackDate'.

I have tried to also create a formula within MS SQL but I come up with
errors. I have done something like this in Access which worked. The
Formula looked like this:
                         IIF(Now()>[DueBackDate],int(Now()-[DueBackDate]),0)

Is there a way to convert this formula so that is would work in MS SQL?

Thanks.