Working with formulas in Word tables - calculating time on scene.

I am creating forms to be filled out on the computer and then emailed to me.
I am trying to create a formula in a table that can tell us how many minutes
(hours and minutes) that our personnel spent on a scene. For example: I
have a box that they can enter their "Response Time" in and then I have
another box where they can enter "Time Clear of Scene". I want a formula in
the following cell to total the amount of hours and minutes they spent at
this certain call. How do I do this? Help? Please??? Thanks in advance!

Christy