DuneBug is exactly right on this one. You will want to do something like this:
DateTime startTime = DateTime.Parse( stringValueOfStartTimeField );
DateTime endTime = DateTime.Parse( stringValueOfEndTimeField );
TimeSpan duration = endTime.Subtract( startTime );
string output = duration.Hours + ":" + duration.Minutes; // Assumes < 24 hours, also probably want to add zero padding
// same thing for mttr
This has been discussed several times on the forum so you might want to do a quick search if you run into trouble.
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge