I want to calculate the difference in time between two fields to get a duration.

Start time 01:10:53
Stop time 01:20:05
Duration 00:09:12

How can I achieve this? Or can I?

You use the DateDiff function.
In the text box where you want
the duration to appear, enter this:
=Datediff("n",[Start time],[Stop time])

The "n" stands for minutes. "s" is seconds, "h" is hours. You can find the complete list under DateDiff Function in Help.