The start time is recorded when you start the stopwatch. This is so the script knows the elapsed time between the current time and when you started the stopwatch. When you pause the stopwatch, then it writes that elapsed value (in number of days) into the duration…so when you start the stopwatch again, it records the new start time and calculates the elapsed time from the new start time to the current time, but also adds the previous duration. They are really used to help the stopwatch script calculate the total elapsed time even when you pause and start the stopwatch multiple times. Stopping the stopwatch clears both values, so the script knows that no time has elapsed.
What exactly are you trying to get out of the duration column? It only holds the value that was elapsed prior to pushing the pause button. It translates to the number of elapsed days where the stopwatch was running prior to pressing pause. How would you expect to display that as a date?