calculating times in a form

I have a Form i cant figure out how to calculate times i have 6 boxes i need to put times into and the 7th box would be the total time.

this form is used at our fire dept to show the trucks en-route -to- on scene time- to- scene secure -to- left scene -to- in service time.. and then the total time in the final box i believe you just subtract the enroute time from the total time. but i have no idea i tried everything and cant figure it out.  i can upload the form if you need.

THANKS