Time
Time zones
It is not the same time everywhere. Travel east and clocks go forward; travel west and they go back.
Why time zones exist
The Earth turns, so the sun rises at different moments in different places. Everyone wants midday to be roughly when the sun is highest. So the world is divided into TIME ZONES, each keeping its own clock time. Within a zone every clock agrees; cross into the next one and the time changes.
Which way do the clocks go?
Travel EAST → clocks go FORWARD (later). Travel WEST → clocks go BACK (earlier). If it is 12:00 in London and you travel east to a zone 3 hours ahead, it is 15:00 there. Travel west to a zone 5 hours behind and it is 07:00. Getting the direction right is most of the work.
Working out the local time
1. Start with the time you know. 2. Add the difference if the other place is ahead; subtract if it is behind. 3. Check whether you have crossed midnight. If it is 22:00 in London and a city is 4 hours ahead, adding 4 gives 26:00 — which is not a real time. Subtract 24 to get 02:00 the NEXT day.
Flight times
The clock time at your destination is not the same as how long you flew. A flight leaving at 09:00 and landing at 15:00 local time, in a place 2 hours ahead, did not take 6 hours. Put both times in the SAME zone first: landing at 15:00 there is 13:00 at home. So the flight lasted 4 hours.
Worked examples
Travelling 3 hours east
- It is 12:00 where you are.
- Travelling east means clocks go forward.
- 12:00 + 3 hours.
Answer: It is 15:00 there
Travelling 5 hours west
- It is 12:00 where you are.
- Travelling west means clocks go back.
- 12:00 − 5 hours.
Answer: It is 07:00 there
Crossing midnight
- It is 22:00 and the other city is 4 hours ahead.
- 22:00 + 4 = 26:00, which is not a real time.
- Subtract 24 hours.
Answer: It is 02:00 the next day
How long was the flight?
- Take off 09:00; land 15:00 local, in a zone 2 hours ahead.
- Convert the landing time to home time: 15:00 − 2 = 13:00.
- 09:00 to 13:00 is 4 hours.
Answer: The flight took 4 hours
Which way do clocks change going east?
- The Earth turns from west to east.
- Places to the east see the sun earlier.
Answer: Clocks go forward
Where you see this in real life
- Phoning family in another country and working out if they are awake.
- Watching a live sports match shown at 3 a.m. because of the time difference.
- Jet lag after a long flight, when your body clock is still in the old zone.
Key words
- Time zone
- A region that keeps the same clock time.
- Ahead
- Later than another time zone.
- Behind
- Earlier than another time zone.
- Local time
- The clock time in a particular place.
- Time difference
- The number of hours between two zones.
- Midnight
- 12:00 at night, when the date changes.