How to get the time difference from dutyoff to dutyon of AM

Question

We're stuck of this function on how to get the total hours of time column, what we want is, the duty off minus the time of duty on from morning time, as well as the afternoon time so that we can sum the hours of the time column. Dutyon and DutyOff are not column names, so we're having a hard time. Can you pleaaaaase help us. And we want to display the total to the last row of time column. Thanks.