Time Zones

The earth is divided into 24 time zones, each of which is 15° of Longitude. The middle line, or meridian, is located in Greenwich in London. All time zones are based around Greenwich Mean Time (GMT). Each time zone to the west of Greenwich is on an earlier time zone, while those to the east of Greenwich are on a later time.

Each time zone is one hour different to the next. Within each zone all clocks are set to the same time. This is the case all over the world - there are very few exceptions, such as Australia, where there are half-hour zones.

Also, the theoretical one hour separation is not universal. The shapes of time zones can be quite irregular because they usually follow the boundaries of states, countries or other administrative areas.

In our How It Works chapter we have used the examples of London (Great Britain), Berlin (Germany) and New York (New York, USA):
- Berlin is located one time zone east of London so it is one hour ahead of GMT. When it is midday in London, it is 1pm in Berlin. This fact has been denoted as (GMT+1).
- New York is located six time zones west of London so it is six hours behind GMT. (GMT-6).
- London is located in the Greenwich Mean Time zone (GMT).