answer:If you are keeping time to the quarter hour, then the decimals are really decimals: i.e., hundredths of an hour. So you don’t need a formula. It’s simple subtraction, the same as you’d use if it were money. All you have to do to compute time after midnight is to add 12.00 to your ending time. 3:30 a.m. = 15.50. Subtract 11.75 from 15.50. This is exactly how I always kept my time sheets. As long as you stick to quarter hours and use decimals, those numbers are great tools for several work-related purposes.