24-Hour Clock Explained Simply (With Easy Conversions)
Train stations, flight boards, and school timetables almost always use the 24-hour clock, which trips students up more than it should, given how simple it actually is once explained properly.
The 12-hour clock repeats itself twice a day, needing AM and PM to tell the difference. The 24-hour clock never repeats — it simply counts from 00:00 at midnight all the way up to 23:59 just before the next midnight.
Converting PM times to the 24-hour clock
For any time from 1pm onwards, add 12 to the hour. 3:00pm becomes 15:00, because 3 + 12 = 15. 8:45pm becomes 20:45, because 8 + 12 = 20.
Converting AM times to the 24-hour clock
Morning times mostly stay the same, just written with two digits — 7:00am becomes 07:00. Midnight is the one exception: 12:00am becomes 00:00, not 12:00, since the 24-hour clock starts counting from zero.
Noon and midnight
Noon, 12:00pm, stays as 12:00 on the 24-hour clock. Midnight, 12:00am, becomes 00:00 — this pair catches out even adults sometimes, so it's worth learning as a special case rather than trying to apply the usual rule.
Converting back to the 12-hour clock
Going the other way, if the 24-hour time is 13:00 or later, subtract 12 and add pm. 15:30 becomes 3:30pm, since 15 − 12 = 3. If the hour is 12, it's already 12pm — no subtracting needed. If the hour is 00, it becomes 12am.
Real-life example: a train timetable
A train timetable shows a departure at 23:50. Converting to the 12-hour clock: 23 − 12 = 11, so that's 11:50pm — nearly midnight.
Practice questions
- Convert 4:00pm to 24-hour time. Answer: 16:00
- Convert 09:00 to 12-hour time. Answer: 9:00am
- Convert 11:45pm to 24-hour time. Answer: 23:45
- Convert 18:30 to 12-hour time. Answer: 6:30pm
- Convert 12:00am to 24-hour time. Answer: 00:00
Common mistakes I see in the classroom
The most common one is forgetting to add 12 for afternoon and evening times, writing 15:00 as "3:00" without converting it back to 12-hour form first, or writing 3:00pm as just "3:00" in 24-hour form, missing the +12 step entirely.
Another frequent mistake is mishandling midnight and noon, treating 12:00am as "12:00" in the 24-hour clock instead of the correct 00:00.
Tips for parents
Digital clocks, microwaves, and oven timers often already display the 24-hour clock at home. Asking your child to read one of these aloud in 12-hour form turns a glance at the oven into quick, everyday practice.
Tips for teachers
I keep a simple table on the wall showing every hour in both 12-hour and 24-hour form side by side. Referring back to it regularly, rather than reteaching the conversion rule each time, builds familiarity faster than repeating the rule alone.
How step-by-step learning tools help
A step-by-step maths tool can show exactly how a time converts between the 12-hour and 24-hour clock, one step at a time, with unlimited fresh times for a child to practise converting at their own pace.
Frequently asked questions
What is the 24-hour clock? It's a way of telling the time that counts all 24 hours of the day in one continuous sequence, from 00:00 at midnight to 23:59 just before the next midnight, without needing AM or PM.
How do you convert PM times to the 24-hour clock? Add 12 to the hour — 5:00pm becomes 17:00, since 5 + 12 = 17.
What does midnight look like on the 24-hour clock? Midnight is written as 00:00, not 24:00, since the 24-hour clock counts up from zero at the very start of each day.
At what age do children learn the 24-hour clock? This is usually introduced around age 9 to 10, once telling the time on a normal 12-hour clock is already secure.
Why do trains and flights use the 24-hour clock? It avoids any confusion between AM and PM, which matters a lot when a journey might start in the morning and finish late at night.
No AM, No PM, Just One Number
The 24-hour clock feels unfamiliar mainly because AM and PM aren't there to lean on. Once a child trusts the simple add-12 or subtract-12 rule, and remembers the two special cases of noon and midnight, reading any timetable becomes far less of a puzzle.