Everyday maths 1 (Wales)
Everyday maths 1 (Wales)

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6 Time

What’s the difference between the 12-hour clock and the 24-hour clock?

12-hour clock24-hour clock

The hours go from 12 to 12, twice a day

You must use ‘a.m.’ or ‘p.m.’: ‘a.m.’ means ‘before noon’ and ‘p.m.’ means ‘after noon’

The hours go from 0 to 23

Time is always shown in four digits

You do not use ‘a.m.’ or ‘p.m.’

Commonly used in timetables, mobile phones and computers

Example: Converting times before noon

It is easy to change from the 12-hour clock to the 24-hour clock for times before noon (or times ending ‘a.m.’). So for example, ‘4:25 a.m.’ would be written as ‘04:25’. ‘11:35 a.m.’ would be ‘11:35’.

(Note that many timetables don’t show the colon in the 24-hour clock, so these times would be shown as ‘0425’ or ‘1135’.)

How would you write a quarter to eight in the morning as a 24-hour clock time?

Method

Fifteen minutes before eight o’clock is the same as 45 minutes past 7, so it is written as 07:45.

To change from the 12-hour clock to 24-hour clock for times after noon (ending ‘p.m.’), you usually need to add 12 hours.

Example: Converting times after noon

  1. How would you express 8:15 p.m. using the 24-hour clock?
  2. How would you write quarter to eight in the evening as a 24-hour clock time?

Method

  1. You need to add 12 hours:

    So 8:15 p.m. is 20:15 in a 24-hour clock.

  2. You add 12 hours:

    So 7:45 p.m. is 19:45 in a 24-hour clock.

Hint: Take care with times with the hour of 12. So quarter past midnight, or 12:15 a.m. is 00:15 in a 24-hour clock. Likewise quarter past midday, or 12:15 p.m., is 12:15 in a 24-hour clock.

Now try the following activity.

Activity 20: The 24-hour clock

Figure 44 A clock face

Express the following times in the 24-hour clock. Remember, for some of the times you will need to use the method of adding on 12 hours to convert the time intothe 24-hour format. You could use Figure 44 to help you count on 12 hours.

  1. 8:15 a.m.
  2. 2:50 p.m.
  3. 5:40 a.m.
  4. 9:22 p.m.
  5. Ten to ten in the morning
  6. Five past six in the evening

Answer

  1. 08:15
  2. 14:50
  3. 05:40
  4. 21:22
  5. 09:50
  6. 18:05

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