There are 60 minutes in an hour and 24 hours in a day. The 24-hour clock counts from 00:00 (midnight) to 23:59. To change pm times, add 12 hours: 3:00 pm = 15:00. Duration is the time between a start and an end, for example 8:15 am to 10:45 am is 2 hours 30 minutes.