How do I add or subtract Times? You can add and subtract times in a few different ways. For example, to get the difference between 8:00 AM - 12:00 PM for payroll purposes you would use: =("12:00 PM"-"8:00 AM")*24, taking the end time minus the start time.Note that Excel calculates times as a fraction of a day, so you need to multiply by 24 to get the total hours.