# Unit 14 Section 2 : 12-hour and 24-hour Clocks

The 24-hour clock system can be used to tell us if times are in the morning or afternoon.
Alternatively, times can be given as a.m. (morning) or p.m. (afternoon).

Think about the time at half past three in the afternoon.

In the 24-hour clock system we give the number of hours since the beginning of the day for the first two digits,
and the number of minutes since the beginning of the hour for the last two digits.
In this example, you have 12 hours in the morning + 3 hours in the afternoon which makes 15 since the start of the day.
There are 30 minutes since the beginning of the hour so the 24-hour clock time is 1530.

We can also specify the part of the day by writing the time in the normal way using digits and adding on a.m. or p.m..
If the time is in the morning we add a.m. and if the time is in the afternoon we add p.m..
In this example, the time is in the afternoon, so it would be written 3:30 p.m..

The time "half past three in the afternoon" can be written as 1530 or 3:30 p.m.

## Example Questions

Example 1
Write these times in 24-hour clock time:
(a) 3:06 a.m.
This is 3 hours from the beginning of the day, so the 24-hour time is 0306.
(b) 8:14 p.m.
This is 20 hours (12 + 8) from the beginning of the day, so the 24-hour time is 2014.

NOTE: The 24-hour clock time will always contain exactly 4 digits.

Example 2
Write these times using digits and a.m. or p.m.:
(a) 1428
This is 14 hours from the beginning of the day, which means it must be in the afternoon.
Taking off the 12 hours in the morning gives 2 hours since midday, so the time is 2:28 p.m.
(b) 0742
This is 7 hours from the beginning of the day, which means it must be in the morning, so the time is 7:42 a.m.

Practice Questions
Work out the answer to each of these questions then click on the button marked to see whether you are correct.

(a) Write the 24-hour clock time 1731 using a.m. or p.m. (b) Write 9:26 a.m. using 24-hour clock time. ## Exercises

Work out the answers to the questions below and fill in the boxes. Click on the button to find out whether you have answered correctly. If you are right then will appear and you should move on to the next question. If appears then your answer is wrong. Click on to clear your original answer and have another go. If you can't work out the right answer then click on to see the answer.

Question 1
Convert these times to 24-hour clock times:

(a) 9:42 a.m.  (b) 11:28 p.m.  (c) 11:14 a.m.  (d) 7:13 p.m.  (e) 9:21 p.m.  (f) 4:18 p.m.  (g) 10:05 a.m.  (h) 12:15 a.m.  (i) 12:15 p.m.  Question 2
Convert these times to 24-hour clock times:

(a) quarter to eight in the morning  (b) ten minutes to midnight  (c) ten past nine in the evening  (d) five to seven in the morning  (e) quarter past five in the afternoon  (f) half past two in the afternoon  Question 3
Write these 24-hour clock times in 12-hour clock times, using a.m. or p.m.

Question 4
Add "a.m." or "p.m." on to the end of each sentence so that it makes sense.

(a) Joshua woke up at 6:45  (b) Miles came home from school at 3:55  (c) Andy started his night shift at 10:15  (d) Grace ate her lunch at 11:45  (e) James went to bed at 8:55  (f) Heidi cooked a meal at 5:15  Question 5
Below are the timings for a school day. Convert them into 24-hour clock times.
 Registration 8:55 a.m.  Lesson 1 9:05 a.m.  Lesson 2 10:05 a.m.  Lesson 3 11:05 a.m.  Lunch 12:05 p.m.  Lesson 4 1:05 p.m.  Lesson 5 2:05 p.m.  Day ends 3:05 p.m.  Question 6
Dave leaves home at 0900 and returns 7 hours later. What time does he get home?    