Each day becomes decimal hours
The tool reads the start and end time you enter for a day, measures the gap in minutes, and writes that shift as a fraction of an hour: 09:00 to 17:30 becomes 8.5. It then adds the seven daily values. A shift that crosses midnight, like 23:00 to 07:00, has to be counted as an overnight span, or the subtraction runs negative and that day poisons the week's total.