The world is divided into 24 time zones.
 By convention, each country determines its time relative to the time at the Meridian Line in Greenwich, England (a suburb of London).
For example in France you say the time is GMT+1, Japan is GMT+9, Brazil is GMT-3.
This means that for France you have to add 1 hour to GMT (Greenwich Mean Time, or the current time at the Greenwich Meridian Line), for Japan you add 9 hours, for Brazil you subtract 3 hours. When it is noon in London, it is 1:00 PM in Paris, 9:00 PM in Tokyo and 9:00 AM in Rio de Janeiro.

Greenwich Mean Time (Suburbs of London) GMT 12:00 France +1 13:00 Japan +9 21:00 Brazil -3 9:00

Just add or subtract the number indicated in order find out the time in each country, when it is 12:00 Noon in Greenwich.

Some countries have several time zones (for example Russia covers 10 time zones, with Moscow being GMT+2 and Vladivostock GMT+12. In these cases, the figures listed would give the time closest to GMT.

Many countries have adopted Daylight Saving Time in order to save energy.
This means they add one hour during certain months (generally from April to September in the Northern Hemisphere and from October to March in the Southern Hemisphere).
The exact dates may vary from one country to another.

Knowing a country’s time difference relative to Greenwich Mean Time allows you to calculate the time difference from one country and to another.
Suppose you’re in country A (at noon) and you want to know the time in country B, then you need to know the time differences for those countries.
A variety of cases may arise, so to illustrate them we’ll put them on a time line:
A Greece, B Thailand, C California, D Peru, E Guadeloupe and F India.

Standard Time and Daylight Saving Time:

 Africa

 Asia

 Canada

 Carabbean Islands

 Europe

 Ex-USSR Republics

 Latin America

 Middle East

 Oceania

 United States