Lesson Guide
Explanation
An ordinary clock face shows each time twice a day. Seven o'clock is breakfast or it is dinner, and the only thing separating them is a tiny am or pm that is easy to miss, mishear or forget to write. For most of life that is fine. For a flight, a train timetable, an army order or a hospital drug chart it is not fine at all, because being twelve hours out could be a disaster. So those places use 24-hour time, where every single time of the day has its own unique number.
Your Turn
Name two times today that would be written the same on an ordinary clock, twelve hours apart. Check: any pair like 3:00 in the morning and 3:00 in the afternoon.
Watch Out
am covers midnight to midday; pm covers midday to midnight. Mixing them up is the whole problem 24-hour time solves.
Explanation
The idea is simple: instead of restarting at 1 after midday, the hours just keep counting. Midnight is 00:00. The morning runs 00:00 to 11:59 and looks exactly like an ordinary clock. Then at midday, instead of going back to 1, it carries on: 12, 13, 14, and so on up to 23. The last minute of the day is 23:59, and then it rolls over to 00:00 again. Twenty-four hours, twenty-four different hour numbers, no am or pm needed.
Your Turn
Count aloud from 11:00 through midday to 15:00 in 24-hour time. Check: 11, 12, 13, 14, 15 — no restart.
Watch Out
There is no 24:00 on a clock. The day ends at 23:59 and the next one starts at 00:00.
Explanation
To turn a pm time into 24-hour time, add 12 to the hours and leave the minutes exactly as they are. One pm becomes 13:00. Half past four in the afternoon becomes 16:30. Quarter to ten at night becomes 21:45. The minutes never move — it is only ever the hour number that changes. Going the other way, from 24-hour time back to pm, you subtract 12: 19:20 is twenty past seven in the evening.
Your Turn
Write 3:15 pm and 8:00 pm in 24-hour time. Check: 15:15 and 20:00.
Watch Out
Do not add 12 to the minutes. 4:30 pm is 16:30, never 16:42.
Explanation
There are exactly two times that break the rule, and they are worth memorising. Twelve am is midnight, and in 24-hour time it is 00:00 — the very start of the day, not the end. Twelve pm is midday, and it simply stays 12:00, because adding 12 would give 24:00 which does not exist. Everything else follows the add-12 rule perfectly. Get these two into your head and the whole system is straightforward.
Your Turn
Write midnight and midday in 24-hour time without looking. Check: 00:00 and 12:00.
Watch Out
12 am is the start of the day and 12 pm is the middle. Almost everyone gets these the wrong way round at first.
Explanation
Timetables are where you will meet this for real. Notice there is no am or pm anywhere on this one — it does not need any, because every time is unique. The 14:05 departure leaves at five past two in the afternoon and arrives at 14:52, which is 2:52 pm. Working out that the trip takes 47 minutes is just the elapsed time you already know how to do, and it is actually easier in 24-hour time because you never have to worry about crossing midday.
Your Turn
From this timetable, how long does the 18:30 service take, and what time is that in pm? Check: 47 minutes, departing 6:30 pm.
Watch Out
When a journey crosses midnight, the arrival time is smaller than the departure. That is normal — it is the next day.
📌 Key Points — this lesson in a nutshell
- 24-hour time gives every time of day a unique number, so no am or pm is needed.
- Morning times are unchanged; after midday the hours keep counting to 23.
- pm → 24-hour: add 12 to the hours, leave the minutes alone.
- Two to memorise: midnight is 00:00, midday is 12:00.
- The day ends at 23:59 — there is no 24:00.
Step 1: Introduction
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