Åk 6–9

2.7 Time Zones
The world is divided up into 24 hour time zones in order to use “normal time”.  Besides the 24 zones, some countries use half time differences.  Zones normally follow the meridians which are the lines running from north to south.

The International Date Line is in the Pacific Ocean, east of New Zealand and West of Hawaii.  This means that there is a difference of one calendar day on each side of the line.

Bild: www.lfv.se

The Prime Meridian goes through England and the city of Greenwich outside of London.  When you want to describe the time difference on Earth you start from the Prime Meridian.  In Sweden, we are an hour before England, in other words when the time is 10:00 in Sweden, it is 9:00 in England.  In relation to the Prime Meridian, called GMT, Greenwich Mean Time, our time is -1.
Normally, zones follow practical lines like national boundaries or coasts.  13 countries are large enough to the west and east that they are divided into several times zones. USA for example has four times zones.

In circa 70 countries, several in Europe, is used summer and winter time.  This is used in order to maximize daylight.  In Sweden, we turn the clocks forwards one hour in the spring and one hour back (to normal time) in the fall.

(Translation note:  One way that has traditionally been used to remember this change has been the saying “spring forward, fall back”.)

Example A

Imagine that you are going to fly from Stockholm to Peking.

The flight leaves on Wednesday 22:30 and you land on Thursday 12:55. How long did the flight take?

1. Begin by calculate the flight time without the time difference.
In this example, it is 14 hours and 25 minutes.

2. Then take away the time difference.
Stockholm – Peking has a 7 hour time difference.

This means that the flight takes 14 h and 25 min – 7h =
(14 - 7) h and (25 - 0) min = 7h and 25 min.

Example B

What is the time difference between New York and Stockholm if you start flying from Stockholm at 10:40 and land the same day 13:30 and the trip took 8 h 55 min?

1. Calculate the flying time using local time: 10:40 – 13:35 is 1h 20 min + 1h 35 min = (1 + 1) h and (20 + 35) min = 2h 55 min.

2. Then take away the time difference from the flight time.
8h 55 min – 2h 55 min = 6h, which is the time difference between Stockholm and New York.