The 24-Hour Clock

Two ways to tell the time

You already know the 12-hour clock: it counts the hours 1 to 12 twice in a day, once before noon and once after. To tell the difference we add am (morning) or pm (afternoon and evening).

The 24-hour clock does something cleverer. Instead of starting again at noon, it keeps counting all the way up. Midnight is hour 00, and the hours run 00, 01, 02 … all the way to 23. Because every hour has its own number, you never need am or pm.

If you are still getting comfortable with reading a clock face, it helps to revise Telling the Time on a Clock first.

How 24-hour time is written

A 24-hour time always has four digits and a colon:

• The first two digits are the hours (00 to 23).
• The last two digits are the minutes (00 to 59).

So we write 09:05, not 9:5. We keep the leading zeros. Some people write the same time as 0905 with no colon, especially in the military, said as "oh nine oh five".

The key landmarks

Moment	12-hour	24-hour
Midnight	12:00 am	00:00
Morning	7:30 am	07:30
Midday (noon)	12:00 pm	12:00
Afternoon	4:00 pm	16:00
Evening	9:15 pm	21:15

Notice two friendly facts: in the morning the hours are almost the same in both systems (you just add a leading zero if needed). It is only in the afternoon and evening that the numbers change.

Converting 12-hour to 24-hour

Here is the simple rule:

• am times: keep the same hours, write four digits. 6:45 am → 06:45. (The one exception: 12 midnight am → 00:00.)
• pm times: add 12 to the hours. 5:20 pm → 5 + 12 = 17 → 17:20. (The one exception: 12 noon pm stays 12:00.)

Worked example 1

Write 8:10 pm on the 24-hour clock.

1. It is pm, so add 12 to the hour: 8 + 12 = 20.
2. Keep the minutes the same: 10.
3. Answer: 20:10.

Converting 24-hour to 12-hour

Now go the other way:

• If the hours are less than 12, it is am (00 becomes 12 am, midnight).
• If the hours are 12 or more, it is pm. If they are bigger than 12, subtract 12.

Worked example 2

Write 19:45 as a 12-hour time.

1. 19 is bigger than 12, so subtract 12: 19 − 12 = 7.
2. Because it was past 12, it is pm.
3. Answer: 7:45 pm.

Why we have a 24-hour clock

Imagine a train timetable that just said "the train leaves at 7:00". Is that breakfast time or bedtime? On a busy railway, that confusion could make you miss your train by twelve hours! The 24-hour clock fixes this: 07:00 can only mean the morning and 19:00 can only mean the evening. That is exactly why timetables, computers and digital clocks use it.

Reading a timetable

Here is part of a bus timetable. Each row is one stop; each column is one bus.

Stop	Bus A	Bus B
High Street	08:15	13:40
Library	08:32	13:57
Station	08:50	14:15

Bus B reaches the Station at 14:15, which is 2:15 pm.

Working out how long a journey takes

To find a duration, count on from the start time to the end time. Reaching the next whole hour first makes this easy.

Worked example 3

A film starts at 18:40 and ends at 21:10. How long is it?

1. From 18:40 to 19:00 is 20 minutes.
2. From 19:00 to 21:00 is 2 hours.
3. From 21:00 to 21:10 is 10 minutes.
4. Add them up: 2 hours + 20 + 10 minutes = 2 hours 30 minutes.

Try it yourself

1. Write these as 24-hour times: 7 am, 1 pm, 9:30 pm, 11:55 pm.
2. Write these as 12-hour times: 06:20, 12:00, 15:45, 23:05.
3. A train leaves at 09:25 and arrives at 11:10. How long is the journey?

(Answers: 07:00, 13:00, 21:30, 23:55; 6:20 am, 12:00 noon, 3:45 pm, 11:05 pm; 1 hour 45 minutes.)

Great job!

You can now switch between the 12-hour and 24-hour clock, read a timetable, and work out durations. Next, practise more time and number skills with Number Patterns and Sequences.