Primary 4 Maths | 24-Hour Clock and Time Calculations Lesson-Prep Guide (for tutors) | 45-minute flow, teaching script, common errors

2026-11-15

Tutor lesson-prep guides · Primary Maths

Time errors are rarely subtraction errors: pupils treat hours and minutes as decimal, as if an hour held 100 minutes. This lesson drills one habit, that borrowing means borrowing 60.

Objectives — By the end of the lesson the pupil can: (1) convert between a.m., p.m. and the 24-hour clock in both directions; (2) add and subtract times with minutes borrowing 60 rather than 10; (3) find a time difference across noon and across midnight; (4) self-check that the finishing time is later than the starting time.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Hours and minutes	says how many minutes are in one hour	answers 60 minutes
Two-digit subtraction	works out 85 minus 40	answers 45
Morning and afternoon	says how many hours after noon is 3 p.m.	answers the third hour

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Entry check	asks the minutes in an hour and 85 minus 40, writes them on the right of the board	answers aloud and names the two units
6-15 min	Convert both ways	shows 3:45 p.m. as 15:45, then 21:20 as 9:20 p.m.	converts two items and says whether 12 is added or subtracted
15-25 min	Difference within one day	shows 09:40 to 11:25 by turning both into minutes	gets 105 minutes and writes it as 1 hour 45 minutes
25-34 min	Across noon and midnight	shows 22:30 to 01:15 by topping up to 24:00 first	gets 165 minutes and writes it as 2 hours 45 minutes
34-42 min	Independent practice	reads questions only and checks who borrows 60	works three items and rechecks each in minutes
42-45 min	Exit check	marks at once	says whether the slip was borrowing or conversion

03 Teaching script (read it out)

Scene 1: Act 1: add 12 for the afternoon

The 24-hour clock runs once, from 00:00 to 23:59. Morning times are copied straight across, so 9 a.m. is 09:00. Afternoon times add 12: 3:45 p.m. becomes 3 plus 12 equals 15, so 15:45. The other way round, 21:20 is bigger than 12, so subtract 12 to get 9, which is 9:20 p.m. One rule decides it every time: an hour number above 12 is afternoon or evening.

To write 3 p.m. on the 24-hour clock, what do you add? And how do you know 21:20 is in the afternoon?

Scene 2: Act 2: subtracting time means borrowing 60

Time is not decimal. One hour is 60 minutes, so when the minutes are too small you borrow 60. From 11:25 take 09:40: 25 minutes cannot cover 40, so borrow one hour to make 10:85, then subtract 09:40 to get 1 hour 45 minutes, which is 105 minutes. Remember the borrow is 60, not 10.

When the minutes are too small, how much do you borrow from the hours?

Scene 3: Act 3: across midnight, split the journey

From 22:30 to 01:15 the clock passes midnight, so the two cannot simply be subtracted. Split it: 22:30 to 24:00 is 1 hour 30 minutes, and 00:00 to 01:15 is 1 hour 15 minutes, giving 2 hours 45 minutes, which is 165 minutes. Splitting is the only reliable method; do not guess by feel.

For a difference across midnight, what time does the first part run up to?

04 Question types and worked steps

Type	Example	Steps	Answer
Morning and afternoon conversion	3:45 p.m. written on the 24-hour clock	add 12 to the hours, keep the minutes	15:45
Reading the 24-hour clock aloud	21:20 as ordinary time	subtract 12 from the hours, keep the minutes	9:20 p.m.
Difference within one day	how long from 09:40 to 11:25	turn both into minutes, subtract, convert back	105 minutes, which is 1 hour 45 minutes
Difference across midnight	how long from 22:30 to 01:15 the next day	split: top up to 24:00, then add the next day	165 minutes, which is 2 hours 45 minutes

Sanity check (teach this): Three checks: the finishing time must be later than the starting time, so a negative or reversed answer means the split across midnight was skipped; the minutes part must stay below 60, so 85 minutes means the conversion is unfinished; and recomputing in minutes must give the same result as the hours-and-minutes form.

05 Independent practice (3 questions)

Question	Answer	What to watch
How many minutes from 07:55 to 08:10?	15 minutes	whether 55 minutes was treated as 5
How long from 11:50 to 12:35?	45 minutes	whether crossing noon confused the subtraction at all
What time is 90 minutes after 10:35?	12:05	whether 90 minutes was written as 1 hour 90 minutes

06 Common mistakes and what to say

Mistake	Student writes	Say this
Borrowing as if time were decimal	11:25 minus 09:40 answered as 1 hour 85 minutes	Borrow 60 in time, so 85 minutes becomes 1 hour 25 minutes.
Subtracting straight across midnight	22:30 to 01:15 answered as 21 hours 15 minutes	Once midnight is crossed, split the journey and top up to 24:00 first.
Forgetting the add 12 in the afternoon	3:45 p.m. written as 03:45	Afternoon adds 12, so 3 plus 12 is 15, giving 15:45.
Leaving the minutes unconverted	an answer of 1 hour 105 minutes	105 minutes is 1 hour 45 minutes; the minutes box never holds 60 or more.

07 Homework (paper, section and question numbers)

Primary 4 maths paper 1: measures and time section — Section C, questions 1 to 3

08 Exit check (last 5 minutes)

Question	Answer	Decision
How long from 08:15 to 17:00?	525 minutes, which is 8 hours 45 minutes	wrong: reteach Act 2, borrow 60
Write 135 minutes in hours and minutes.	2 hours 15 minutes	wrong: reteach the hours and minutes conversion
What time is 50 minutes before 15:20?	14:30	wrong: reteach borrowing when counting backwards

09 Tutor FAQ

Which year group is this for?

Hong Kong primary schools expect pupils to read and write the 24-hour clock in Primary 4 and to find differences across noon and midnight. Primary 3 stays with whole and half hours, and speed questions in Primary 5 and 6 depend on this conversion.

Is 45 minutes enough?

Yes, but the midnight act needs its full nine minutes. Pupils resist the idea that the two times cannot be subtracted directly, so draw a timeline with both parts and add 165 minutes by hand.

Is there a practice paper?

Yes. The homework line links to the Primary 4 paper with the section and question numbers, and the page carries an answer version.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.