Hi stct
I dont believe you can use task fields in a resource view and vice versa.
What are you trying to achieve?
I have used two custom date fields to put both dates in then used a custom number field to subtract one from the other and it worked fine.
When you say the result was incorrect bear in mind that Project also used hours and minutes in the date. So you will invariably get a result to two decimal places.
Hope this helps.
Tony