Seemed like a simple request!! But after an hours worth of research, this is the best I could come up with.
First off, I would not recommend changing the working time for the project calendar, this could impact all the other task dates in your project.
Here is what I did:
1. Create a new calendar (Tools, Change Working Time, New, Rename to calendar1, make all Saturdays and Sundays Non-Default Working Time-Click on the entire column, OK)
2. Insert Duration1 field
3. Customize Duration1 - Use this formula, also enable formula for summary task.
4. Duration1 will now calculate the total days between start and finish.
Hope this helps!
J Black
PS: I truely believe there is an easier way, but could not find it!!!