A time zone is any of the twenty-four divisions of the Earth's surface used to determine a uniform local standard time for legal, commercial and social purposes. Each of the zones is about 15° of longitude in width (with local variations) and tends to follow the boundaries of countries and their subdivisions.
The 1884 International Meridian Conference, which met in Washington, D.C. at the behest of the President of the United States, established the imaginary prime meridian as the starting reference line for time zones. The meridian (longitude of 0°) passes through Greenwich, England, which is the site of the Royal Greenwich Observatory founded in 1675.
Greenwich Mean Time (GMT) is named after the city and observatory in England. Local time is one hour ahead for each zone east of the prime meridian and one hour behind for each one west of it. Central European Time, or CET, has also been adopted by Western European countries. It is one hour ahead of GMT.
Since 1972 all official time services have broadcast radio signals synchronized to UTC (Universal Coordinated Time). UTC is based on an atomic clock that includes leap seconds. Many countries now legally define their standard time relative to UTC, although some still legally refer to GMT, including the United Kingdom itself. UTC, also called Zulu, is used around the globe by astronomers.
International Date Line
The International Date Line, also established at the International Meridian Conference of 1884, is an imaginary line on the earth that extends from the North to the South Pole through the Pacific Ocean. It is located roughly along the 180th meridian. The line is where each new calendar day officially begins. The day to the east of the line is one day earlier than it is to the west of it.
Many countries have adopted daylight saving time (DST) during part of the year. This typically involves advancing clocks by an hour near the start of spring and adjusting clocks back an hour in the autumn. Modern DST was first proposed in 1907 and was in widespread use in 1916 as a wartime measure aimed at conserving coal. Despite its controversies many countries use DST.
The United States and its territories have ten time zones.
China is the largest country with only one zone. China also has the widest spanning zone in terms of land. Before 1949 China was separated into five time zones.
There are twenty-two places in the world where three or more time zones meet. One example of this is where the borders of Finland, Norway and Russia meet.
Most countries around the equator do not observe DST since the seasonal difference in sunlight is minimal.