If in a query you would use:
MyNewFieldName: DateDiff ("d", [date approved], [date received])
If on a form as a control source:
=DateDiff ("d", [date approved], [date received])
In VBA from an event
Me.YourTextBoxNameHere = DateDiff ("d", Me.[date approved], Me.[date received])
So, perhaps I didn't catch your actual use. If so, let me know so I can go from here.