ok Lynsey - lets get this nailed!
You've got a [StartDate] and a [StopDate] and you want the difference to be in another field - yes?
Firstly - why do you need to save the calculated data? It can be calculated when you need it in queries etc.
Secondly - If you write some code on exit of the [StopDate] - you can do the calulation there then add result to populate the Diff box.
Your code would be something like
Me.DiffBox = DateDiff("d",[StartDate],[StopDate])
Hope this helps
Col