12-hour and 24-hour Time Calculations

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1 Getting Started
2 Watch and Notice
3 Try It Together
4 Build the Table in Your Copy
5 Class Challenge
6 What Did We Notice?
7 What's Next
Pupil practice

1 - Getting Started ~4 mins

Illustration for Getting Started Here is a train-station departure board reading 16:45, and beside it a kitchen clock reading quarter to five in the afternoon (it is after dinner, not before breakfast).

Are these two clocks showing the same time, or two different times? What makes you think so?

Teacher Notes

Take three hands-up answers, not open call-outs. The split you want to surface: some pupils will read 16:45 as completely separate from 'quarter to five', others will spot the link. Don't resolve it yet — that is what Watch and Notice does.

👇 Mark your steps as done as you complete them.

+5 XP

2 - Watch and Notice ~9 mins

The clocks show the same times written two ways. Watch how each morning and afternoon time turns into twenty-four-hour time, and keep an eye on the inner 13 to 24 ring.

Teacher Notes

Point at the inner 13 to 24 ring and the digital readout on each clock.

07:20 (7:20 a.m.): a.m. hours never change, just add the leading zero.

13:15 (1:15 p.m.): after midday keep counting on; ask what one o'clock becomes, one plus twelve is thirteen.

21:50 (9:50 p.m.): same add-twelve move, nine plus twelve is twenty-one.

Midnight and noon catch everyone out, don't rush it. Midnight 12:00 a.m. is 00:00 (day restarts from zero, not 24:00); noon 12:00 p.m. stays 12:00.

Ask the class to say midnight and noon back before moving on.

👇 Mark your steps as done as you complete them.

+10 XP

3 - Try It Together ~11 mins

Today we work through these times together on the clock: 8:30 a.m., 3:45 p.m., 11:10 p.m. and 6:05 a.m. One pupil at a time sets each one on the board. Everyone else works out the 24-hour form in your head and we say it aloud together to check before the clock confirms it.

Set the clock, say the 24-hour time

Teacher Notes

This round is for talking it through together — one pupil sets the clock at the board while the rest of the class works out the 24-hour form in their heads and confirms aloud.

Call one pupil up per time. Have them set the clock, then the class states the 24-hour form before the digital readout is read aloud to confirm. Watch for the p.m. slip — pupils who forget to add twelve will say 3:45 instead of 15:45. Revoice the rule each time: 'after midday, add twelve'. Rotate four pupils so the morning-stays / afternoon-add pattern is seen both ways.

+15 XP

4 - Build the Table in Your Copy ~3 mins

COPYBOOK MOMENT

Illustration for Build the Table in Your Copy In your maths copy, draw a two-column table headed 12-hour and 24-hour. Write each of these times in both forms, one row under the other:

7:20 a.m.
1:15 p.m.
9:50 p.m.
8:30 a.m.
3:45 p.m.
11:10 p.m.

Then underline every row where you had to add twelve.

Teacher Notes

Walk the room glancing at the leading zeros and the underlined afternoon rows — this is whole-class copybook practice, not marking. The common slip is writing 7:20 a.m. as 7:20 with no leading zero.

+10 XP

5 - Class Challenge ~8 mins

First we learn how to go backwards from 24-hour time. Take the station board time 16:45: the hour is bigger than twelve, so subtract twelve from sixteen to get four, the minutes stay 45, and it is afternoon, so that is 4:45 p.m. Morning times under twelve just drop the leading zero and stay a.m.

Now we convert these five times. The first four are already shown in 24-hour form, so for each one say what it is in 12-hour form: 08:05, 14:30, 23:59 and 00:15. The last one goes the other way: 'twenty to midnight', say what that is in 24-hour form. The midnight and midday ones catch people out, so we will say each one aloud before we check it.

Convert the time

Teacher Notes

Before any pupil comes up, work the reverse move on the board with 16:45. Steps: hour is past twelve, subtract twelve (sixteen minus twelve is four), minutes stay 45, label p.m. Result 4:45 p.m. Revoice: 'after twelve, subtract twelve and write p.m.' Morning times under 12:00 just drop the leading zero.

Then the practice questions, pupils take turns at the board, check each answer, and the class confirms before moving on. Keep the board work brisk rather than over-explaining.

The practice set rises in difficulty: 08:05 is a straight morning read (8:05 a.m.), 14:30 needs the subtract-twelve move back to 2:30 p.m., 23:59 is a tricky late-evening one (11:59 p.m.), and 00:15 is the midnight special case (a quarter past midnight, NOT a quarter past noon). The final item, 'twenty to midnight', goes the other direction, asking pupils to reason from words to 23:40. Watch for pupils reading 00:15 as 12:15 p.m.; revoice 'zero-zero hours is midnight'. Also watch for pupils who subtract twelve from morning times like 08:05.

+15 XP

6 - What Did We Notice? ~2 mins

MATHS TALK

Why do airports and railways use the 24-hour clock instead of a.m. and p.m.? What problem could it cause if a train timetable just said '5:00'?

Teacher Notes

Listen for pupils naming the ambiguity of a.m./p.m. — '5:00' could be morning or evening, but '17:00' can only mean one thing. Revoice a strong answer: 'so the 24-hour clock never leaves any doubt about which 5 o'clock you mean'. Head off the idea that 24-hour time is 'harder' — it is actually clearer because the hour number does all the work.

+10 XP

7 - What's Next ~3 mins

Today we learned

24-hour time runs from 00:00 at midnight to 23:59, with no a.m. or p.m.
After midday, add twelve to the hour: 1:15 p.m. becomes 13:15.
Midnight is 00:00, but noon stays 12:00 — the two cases that trip people up.

Coming up

Next we use these time skills to work out elapsed time — how long something lasts when it runs across hours and even across midnight.

Teacher Notes

Keep the recap brisk. The bridge to the next lesson is that knowing 24-hour time makes elapsed-time calculations much cleaner.

+5 XP

Pupil practice

Module 6 · Time and Money Shape & Space

Lesson 60 · 12-hour and 24-hour Time Calculations

Download Activity Book page (PDF)

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Teacher Class Feed

1 - Getting Started ~4 mins

2 - Watch and Notice ~9 mins

3 - Try It Together ~11 mins

Set the clock, say the 24-hour time

4 - Build the Table in Your Copy ~3 mins

5 - Class Challenge ~8 mins

Convert the time

6 - What Did We Notice? ~2 mins

7 - What's Next ~3 mins

Today we learned

Unlock the full learning experience

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