How can I get days overdue to stop revving without setting the value to null

I added in a new Date Field "Date Delivered" and set the form action to set the date to date status was changed to Deliver.  I than added in the formula specified to the field "Days Overdue" .  I now get the error "Expecting duration but found number" and the days overdue field end up being empty.
the field Date Delivered is a date field, and Calculated Finish Date is a formula-Work Date field