Setting up a table with a calculated column
Why not just a smallint to represent "minutes"? 
This is easily translated or formatted (in client code) or SUMmed etc

The actual conversion to minutes can be done in client code or as a DATEDIFF between start and end. I wouldn't use time personally for this because it won't process well (eg can't have &gt; 23:59:59.9999999 if you want to see the monthly time