Because I didn't specify which is 3 o'clock and which is 5 o'clock, I wrote down all the possibilities.

①: 24-hour system: 5-3=2 hours

②: 12 hour system: 3: 00 am to 5: 00 pm, 24-hour system at 5: 00 pm =17: 00, 17-3= 14 hours.

3 pm to 5 am the next day: 12-3+5= 14 hours.