Input Tracked Hours as Start Time / Duration

We often input billing hours based on cell phone logs which give us call time and duration (in minutes, not fractional hours).  It saves some mental math (and opportunity for human error) to be able to input Start Time / Duration and have it figure out the Stop Time.  I think the simplest implementation to code would be to add an input box just above "Hours" and below "Stop Time" labelled "Duration" where you can input the duration in minutes (or maybe hours + minutes), hit a "Calculate" button next to it, and have it populate the Stop Time and Hours fields.