Step-by-step Maths Level 4 · Time to the minute · Lesson 8.2

Step-by-step Maths Level 4: The 12-hour and 24-hour clock

Part of the Step-by-step Maths Level 4 time to the minute chapter. Chapter overview · whole chapter on one page

8.2

The 12-hour and 24-hour clock

Level 4
Say and write a time with a.m. or p.m., and on a 24-hour clock

A 12-hour clock uses a.m. (the morning) and p.m. (the afternoon and evening). A 24-hour clock counts from 00:00 to 23:59.

Key ideas

a.m. is the morning; p.m. is after 12 noon.
On a 24-hour clock, add 12 to the hour for p.m. times.
The minutes stay the same.

Worked example 8.2.1

This clock shows the time in the afternoon or evening. Write it on a 24-hour clock with two digits for the hour.

Method. A 24-hour clock counts from 00:00 to 23:59. Add 12 to the hour for p.m. times.

Note the time of day. p.m. is after 12 noon.
p.m.
Change the hour. For p.m. times, add 12 to the hour.
3 + 12 = 15
Write the time. Keep two digits for the hour and minutes.
15:45

Check. Check: 15:45 is 3:45 in the afternoon.

Watch out. 12:30 p.m. is 12:30, not 00:30.

Worked example 8.2.2

Write 20:30 as a 12-hour time, with a.m. or p.m.

Method. Convert between the 12-hour and 24-hour clock. The minutes stay the same.

Find the hour on a 12-hour clock. Subtract 12 from the hour if it is after 12.
20 - 12 = 8
Decide a.m. or p.m. Times from 12:00 onwards are p.m.
p.m.
Write the time. Write the hour, the minutes, then p.m.
8:30 p.m.

Check. Check: 8:30 p.m. is 20:30 on a 24-hour clock.

Watch out. The minutes never change.

Worked example 8.2.3

This clock shows a time in the morning. Write it on a 24-hour clock with two digits for the hour.

Method. A 24-hour clock counts from 00:00 to 23:59. For a.m. times, keep the hour.

Note the time of day. a.m. is the morning.
a.m.
Change the hour. For a.m. times, the hour stays the same.
8 stays 8, shown as 08
Write the time. Use two digits for the hour.
08:10

Check. Check: 08:10 is 8:10 in the morning.

Watch out. Midnight is 00:00.

Watch out

Watch out. 12:30 p.m. is 12:30 on a 24-hour clock, not 00:30. Only the hours before 12 get 12 added.

Exam tip

Grown-up tip. Ask: “Is it morning or afternoon? Add 12 for an afternoon time.”

Exercise 8B Practice: 12-hour and 24-hour times

Non-calculator

FThis clock shows the time in the morning. Write it on a 24-hour clock with two digits for the hour.

Done

FThis clock shows the time in the morning. Write it on a 24-hour clock with two digits for the hour.

Done

FThis clock shows the time in the morning. Write it on a 24-hour clock with two digits for the hour.

Done

FThis clock shows the time in the afternoon or evening. Write it on a 24-hour clock with two digits for the hour.

Done

FWrite 8:33 a.m. as a 24-hour time.

Done

FWrite 5:57 a.m. as a 24-hour time.

Done

FThis clock shows the time in the afternoon or evening. Write it on a 24-hour clock with two digits for the hour.

Done

FWrite 08:20 as a 12-hour time, with a.m. or p.m.

Done

FThis clock shows the time in the afternoon or evening. Write it on a 24-hour clock with two digits for the hour.

Done

FThis clock shows the time in the morning. Write it on a 24-hour clock with two digits for the hour.

Done

FWrite 4:24 a.m. as a 24-hour time.

Done

FThis clock shows the time in the morning. Write it on a 24-hour clock with two digits for the hour.

Done

FWrite 3:21 a.m. as a 24-hour time.

Done

FWrite 6:27 a.m. as a 24-hour time.

Done

FThis clock shows the time in the morning. Write it on a 24-hour clock with two digits for the hour.

Done

FWrite 9:08 p.m. as a 24-hour time.

Done