Time, duration & time zones
Every clock time is really just a count of minutes since midnight. Once you see a time that way, the four big skills — reading 24-hour time, finding how long something lasts, working out a finish time, and jumping between time zones — are all simple adding and subtracting.
12-hour and 24-hour time
The 24-hour clock counts straight from 00:00 (midnight) to 23:59, never repeating, so there is no need for am or pm. To turn an afternoon or evening time into 24-hour form, add 12 to the hour: 3:45 pm becomes 15:45. Morning (am) times keep their number but are written with two digits, so 9:20 am is 09:20. Going the other way, subtract 12 from any hour of 13 or more and add pm: 20:10 is 8:10 pm. Midnight is 00:00 (12 am) and noon is 12:00 (12 pm).
How long does it last? (duration)
To find a duration, count from the start time to the end time. From 14:30 to 16:15, the whole hours 14:30 → 16:30 are 2 hours (120 minutes); 16:15 is 15 minutes earlier, so the duration is 105 minutes. A handy trick is to step up to the next whole hour first, then count the rest — 11:45 to 12:45 is 60 minutes, then 12:45 to 13:20 adds 35, giving 95 minutes.
Crossing midnight
When the end time is earlier on the clock than the start, the event ran overnight. Count up to midnight, then on from midnight. A flight leaving at 22:30 and landing at 06:15 takes 90 minutes to reach midnight plus 375 minutes after it — 465 minutes in all. The same idea works when adding a duration: 21:40 plus 2 hours 50 minutes passes midnight to land at 00:30.
Finish times
To find a finish or arrival time, add the duration to the start. A train leaving at 10:50 on a 35-minute trip arrives at 11:25 — adding 10 minutes reaches 11:00, then 25 more minutes makes 11:25. Build up to the next hour, then add what is left over.
Time zones
Different places run their clocks ahead of or behind each other by a whole number of hours. "Ahead" means later, so you add; "behind" means earlier, so you subtract. When it is 14:00 in Perth and Sydney is 2 hours ahead, Sydney reads 16:00. When Johannesburg is 6 hours behind Perth, 08:00 in Perth is 02:00 in Johannesburg. If adding or subtracting pushes you past 24:00 or below 00:00, wrap around the clock into the next or previous day.
Typing your answer
For a clock time, type it however is clearest — 15:45, 1545, or 3:45 pm are all accepted. For a duration, just type the number of minutes (or hours when the question asks for hours).
Practice
Warming up your practice…