Unit 13 Section 2 : Time
In this section we revise the use of the 24hour clock and consider problems involving time and timezones.
12Hour and 24Hour clock
Below is a table which will help you convert between 12hour clock and 24hour clock:
As a general rule, the minutes always stay the same between 12hour and 24hour clock.
To convert from 12hour to 24hour
 If the time is 'pm', add on 12 to the hours.
 If the time is 'am' and the hours are '12', change them to '00'
 Remove the ':' in the middle and the 'am' or 'pm at the end.
 Check you have written down four digits  put a '0' in front if there are only three.
To convert from 24hour to 12hour
 If the hours are '12' or more, put 'pm' at the end, otherwise put 'am'.
 If the hours are '13' or more, subtract 12 from the hours.
 If the hours are '00' change them to '12'.
 Add in the ':' in the middle and remove any '0's at the beginning.
Example Question 1
Convert the following times to 24hour clock:
(a) 4:35pm
In this example, add on 12 to the hours because it is 'pm' and remove the ':' and the 'pm'. 
The answer is 1635. 
(b) 12:45am
In this example, change the hours to '00' because it is 'am' and the hours are '12'.
Then remove the ':' and the 'am'. 
The answer is 0045. 
Example Question 2
Convert the following times to 12hour clock:
(a) 1927
In this example, add 'pm' on the end because the hours are 12 or more.
Then take off 12 from the hours because they are 13 or more.
Finally add in the ':' between the hours and minutes. 
The answer is 7:27pm. 
(b) 0457
In this example, add 'am' on the end because the hours are less than 12.
Then add in the ':' between the hours and minutes and remove the '0' at the beginning. 
The answer is 4:57am. 
Example Question 3
Jimmy leaves Exeter at 1345 and arrives in Bristol at 1605. How long was his journey?
Method 1
From 1345 to 1545 is 2 hours.
From 1545 to 1605 is 20 minutes.
We add these together to get the total time. 
The answer is 2 hours 20 minutes. 
Method 2
From 1345 to 1400 is 15 minutes.
From 1400 to 1600 is 2 hours.
From 1600 to 1605 is 5 minutes.
We add these together to get the total time. 
The answer is still 2 hours 20 minutes. 
Time Zones
In the UK, we often talk about countries being 'ahead of us' or 'behind us' in time.
This is because the earth is spinning, so the day starts before or after the UK in different places around the world.
Look at the map below which shows some time zones:
New York is five hours behind the UK. This means that their day doesn't start until five hours after the UK.
If it is 11:30am in the UK, then the time in New York will be 6:30am (to get this, you take off five hours).
Italy is one hour ahead of the UK. This means that their day starts one hour before the UK.
If it is 11:30am in the UK, then the time in Spain will be 12:30pm (to get this, you add on one hour).
General Rule
 If you are travelling East to a country which is ahead of your own, you will add hours to the time.
 If you are travelling West to a country which is behind your own, you will subtract hours from the time.
Example Question
Bermuda is 4 hours behind the UK.
(a) If it is 2:25pm in the UK, what time is it in Bemuda.
We subtract 4 hours because Bermuda is behind the UK. 
The answer is 10:25am. 
(b) If it is 1641 in Bermuda, what time is it in the UK?
If Bermuda is behind the UK then the UK is ahead of Bermuda.
As a result we add on 4 hours when travelling from Bermuda to the UK. 
The answer is 2041. 
Practice Questions
Work out the answer to each of these questions then click on the button marked
to see whether you are correct.
Convert 11:45pm into 24hour clock format.
Convert 3:52am into 24hour clock format.
Convert 0915 into 12hour clock format.
Convert 0058 into 12hour clock format.
Molly leaves Huddersfield at 1322 and arrives in London at 1805.
How long does her journey take?
The time in the United Arab Emirates is 4 hours ahead of the UK time.
What is the time in the United Arab Emirates if it is 3:00pm in the UK?
The UK is 2 hours behind Croatia.
What time is it in the UK if it is 2137 in Croatia?
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.
You have now completed Unit 13 Section 2
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Produced by A.J. Reynolds May 2008
