Help with null field dates (no date)

Hi,

I am working on a report for overdue preventative maintenance services for units in our dept.

To determine the days overdue, I compared the "completed date" against the "due date" using the DateDiff function.

Problem:

Some of the records do not have a "completed date" (work has not begun), but the due date has long past.
How to I tell Crystal that if the "completed date" field is null "empty" then to use todays date (to calculate the number of days past since the due date)????

Any help or guidance would be greatly appreciated!

Thank you,

Helen