Adding subtracting times

I am trying to add and substract start times and need a little help. I have it formatted so the user can enter the numbers without entering the colon. Here is the problem:

start :30
stop 1:55
start 3:00
stop 3:45
Total run 1:70


I have the cells formated, ##":", so the user can enter 0030 and get :30. So then I want to find the total run, I get 1:70. How can I get it cnverted to a time format?