Subtraction of Time Fields in 24Hr using FormCalc

HELP!!
I am trying to use LiveCycle to design a template that incorporates 2 time fields. I am trying to subtract these times so that I know the time that has elapsed (in 24hr format).
The fields I have are: -
Time Down
Time Up
Total Dive Time
these have been formatted so that the input is HH:MM

Can you suggest the easiest way to enable the calculaiton of Total Dive Time?

thanks