12-hour and 24-hour clocks

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12-hour and 24-hour clocks

A boy with a thought bubble above his head. In it are a 12-hour analogue clock and a 24-hour digital clock.

An analogue clock, a clock with hands, shows the numbers 1 through to 12. It is a 12-hour clock.

The first 12 hours of the day, from midnight to midday, are called am.

The next 12 hours, from midday to midnight, are called pm.

However, you can tell the time without saying am and pm. To do this you can use the 24-hour clock.

The 24-hour clock always uses 4 digits. You see this most often on digital clocks.

A boy with a thought bubble above his head. In it are a 12-hour analogue clock and a 24-hour digital clock.
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Activity: The 12 and 24 hour clock

Complete this interactive activity to understand the 12 and 24-hour clock. Then put your knowledge to the test.

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Am and pm

There are lots of different types of clocks.

Some clocks have numbers and moving hands. Some clocks just have numbers.

A 12-hour clock reads 9 o'clock. A digital clock right reads 20 past 10. A digital alarm clock reads 8 o'clock. A 12-hour alarm clock right reads 3 o'clock.

There are 24 hours in 1 day.

Analogue clocks only show 12 hour intervals, so the hour hand must go around twice within a day.

On a 12-hour clock, times can look the same but are different depending on whether they happen in the morning (am) or in the afternoon or evening (pm)

The hands of a 12 hour clock travelling round a clock face twice in a day. As well as a chart showing 24 hour day and points for midnight and midday.

The first 12 hours of the day, from midnight to midday, are 'am'.

The last 12 hours, from midday to midnight, are 'pm'.

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The 24-hour clock

A 24-hour clock doesn’t split the day into two halves. Instead, it keeps counting past 12 and shows all the hours in a day.

Here are some examples:

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  1. A 12 hour clock reads 12 o'clock and a 24-hour clock reads 00:00.

    12 am at night on a 12-hour clock is 00:00 on a 24-hour clock.

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Changing a 12-hour PM time to a 24-hour time

To change a 12-hour pm time to a 24-hour time, add 12 to the hours.

For example, 3.00 pm becomes 15:00.

3 + 12 = 15

A 12-hour clock reads 3 o'clock and a 24-hour clock reads 15:00. In between the clocks there is a line with plus 12 written underneath it.
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Changing a 24-hour afternoon, evening or night time to a 24-hour time

To change a 24-hour afternoon, evening or night time to a 12-hour clock time, subtract 12 from the hours.

For example, 18:00 becomes 6.00 pm.

18 - 12 = 6

A 24-hour clock reads 18:00 and a 12-hour clock reads 6 o'clock. There is a line between them with a minus 12 written underneath it.
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Example 1

A 12 hour clock reading 4 o'clock

Here is an analogue clock showing 4.00.

The time could be either 4:00 am or 4:00 pm. You cannot tell just by looking at a 12-hour clock.

How can you tell if the time is am or pm?

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Example 2

A 24 hour clock that reads 14:30.

Here is a digital clock showing 14:30.

What time is this on the 12-hour clock?

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Example 3

A 12-hour clock reads quarter past 7.

If it’s 7.15 pm on a 12-hour clock. What is this time on a 24-hour clock?

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