Working with time and time zones · Usually met around P6
Time zones
Ahead: add. Behind: subtract. Past midnight, change the day.
How to do it
Method
Ahead: add. Behind: subtract. Past midnight, change the day
15:00 in Hong Kong is 07:00 in London (8 hours behind)
Another way
Count along a time line
15 − 8 = 7
In full
Different places on Earth keep different times. A city “ahead” of Hong Kong is later, so add; a city “behind” it is earlier, so subtract. London is 8 hours behind Hong Kong (7 in summer): 15:00 in Hong Kong is 15:00 − 8 hours = 07:00 in London; Tokyo is 1 hour ahead, so 16:00 at that moment. The 24-hour clock makes it easiest. Going past 24:00 means the next day, so take off 24 hours: at 22:00 in Hong Kong, Sydney (3 hours ahead) is at 01:00 the next day. When you can't subtract, add 24 hours first and go back a day: 03:00 in Hong Kong is 19:00 the day before in London (27 − 8 = 19). For a flight, add the flying time to the departure time first, then change it into local time. Another way: count the hours forwards or backwards along a time line from Hong Kong's time.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
It is 17:55 in Hong Kong. Sydney is 3 hours ahead of Hong Kong. In Sydney it is □ hours □ minutes (24-hour clock).
Fill in each box
Show the answer and working
Answer: 20，55
Ahead means add: 17:55 + 3 hours = 20:55.
Example 2
It is 20:10 in Hong Kong. London is 7 hours behind Hong Kong. In London it is □ hours □ minutes (24-hour clock).
Fill in each box
Show the answer and working
Answer: 13，10
Behind means subtract: 20:10 − 7 hours = 13:10.
Example 3
It is 22:55 on Saturday in Hong Kong. New York is 13 hours behind Hong Kong. What day is it in New York?
- A. Thursday
- B. Saturday
- C. Sunday
- D. Friday
Choose the right answer
Show the answer and working
Answer: B. Saturday
New York's time is Hong Kong's minus 13 hours: 09:55; going back past midnight means the day before.
Practise it in the game →Working with time and time zones · Free to start, unlimited questions, no download