A “time zone” refers generally to any of the 24 regions of the Earth’s surface, loosely divided by longitude, in which standard time is kept.
The Greenwich Meridian, also known as the prime meridian or International Meridian, bisects the primary division of time zones. Each time zone is basically 15 degrees of longitude in width, with local variations, and observes a clock time one hour earlier than the zone immediately to the east.
Time zones’ boundaries are irregular due to political factors, and so this has been a subject of criticism. Some geographically large countries, such as India and China, use only one time zone but other countries, such as Russia and the United States, have more than one time zone.
Each time zone is then theoretically 15 degrees wide, reflecting a one-hour difference in mean solar time.