# Unit 13 Section 2 : Time

In this section we revise the use of the 24-hour clock and consider problems involving time and timezones.

## 12-Hour and 24-Hour clock

Below is a table which will help you convert between 12-hour clock and 24-hour clock: As a general rule, the minutes always stay the same between 12-hour and 24-hour clock.

To convert from 12-hour to 24-hour

1. If the time is 'pm', add on 12 to the hours.
2. If the time is 'am' and the hours are '12', change them to '00'
3. Remove the ':' in the middle and the 'am' or 'pm at the end.
4. Check you have written down four digits - put a '0' in front if there are only three.
To convert from 24-hour to 12-hour
1. If the hours are '12' or more, put 'pm' at the end, otherwise put 'am'.
2. If the hours are '13' or more, subtract 12 from the hours.
3. If the hours are '00' change them to '12'.
4. Add in the ':' in the middle and remove any '0's at the beginning.

Example Question 1

Convert the following times to 24-hour 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 12-hour 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 24-hour clock format.

Convert 3:52am into 24-hour clock format.

Convert 0915 into 12-hour clock format.

Convert 0058 into 12-hour 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.

Question 1
Convert the following tomes to 24-hour clock format:
 (a) 6:45am (b) 6:45pm (c) 2:20pm (d) 11:40pm (e) 10:30am (f) 10:15pm
Question 2
Write the following times in 12-hour clock format. You should write your answers with no spaces like this:
 (a) 1642 (b) 0832 (c) 1042 (d) 2236 (e) 2318 (f) 1520
Question 3
Decide whether the 24-hour clock times below are possible or not possible.
 (a) 1372 Possible Not possible (b) 1758 Possible Not possible (c) 2302 Possible Not possible (d) 2536 Possible Not possible
Question 4
David gets on a train at 0845 and gets off at 1132.

For how long is he on the train?
hours and minutes

Question 5
A journey starts at 1532 and ends at 1830.

How long does the journey take?
hours and minutes

Question 6
Marco boards a ferry at 1842 and gets off at 0633 the next day.

For how long is he on the ferry?
hours and minutes

Question 7
In Venezuela the time is 4 hours behind the time in the UK. Please write your answers using the same format as in previous questions.

(a) What is the time in Venezuela when it is 3:00pm in the UK?

(b) What is the time in the UK when it is 2:30pm in Venezuela?

(c) What is the time in the UK when it is 11:15pm in Venezuela?

Question 8
The time in Norway is 1 hour ahead of the UK. It takes 3½ hours to fly from the UK to Norway.

(a) A plane leaves the UK at 10:15am (UK time). What is the time in Norway when it lands there?

(b) The plane flies back and lands in the UK at 7:22pm (UK time). What was the time in Norway when the plane left?

You have now completed Unit 13 Section 2
 Your overall score for this section is Correct Answers You answered questions correctly out of the questions in this section. Incorrect Answers There were questions where you used the Tell Me button. There were questions with wrong answers. There were questions you didn't attempt.
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