What is Time Zone

A time zone is a region on Earth that uses a uniform time. They are often based on the boundaries of countries or lines of longitude. Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory located in Greenwich, London, considered to be located at a longitude of zero degrees. Although GMT and Coordinated Universal Time (UTC) essentially reflect the same time, GMT is a time zone, while UTC is a time standard that is used as a basis for civil time and time zones worldwide. Although GMT used to be a time standard, it is now mainly used as the time zone for certain countries in Africa and Western Europe. UTC, which is based on highly precise atomic clocks and the Earth's rotation, is the new standard of today.

UTC is not dependent on daylight saving time (DST), though some countries switch between time zones during their DST period, such as the United Kingdom using British Summer Time in the summer months.

Most time zones that are on land are offset from UTC. UTC breaks time into days, hours, minutes, and seconds, where days are usually defined in terms of the Gregorian calendar. Generally, time zones are defined as + or - an integer number of hours in relation to UTC; for example, UTC-05:00, UTC+08:00, and so on. UTC offset can range from UTC-12:00 to UTC+14:00. Most commonly, UTC is offset by an hour, but in some cases, the offset can be a half-hour or quarter-hour, such as in the case of UTC+06:30 and UTC+12:45.

World Time Zone Map

Time zones throughout the world vary, and used to vary even more than they currently do. It wasn't until 1929 before most countries adopted hourly time zones. Nepal, the final holdout, did not adopt a standard offset of UTC until 1956, and even then, has a less common offset of UTC+5:45.

Generally, a time change of 1 hour is required with each 15° change of longitude, but this does not necessarily always happen. For example, China and India only use a single time zone even though they are countries that encompass far larger an area than 15° of longitude. Russia on the other hand, is divided into 11 time zones, though at one point this was reduced to 9 time zones. As can be seen, although there is a general standard throughout the world for defining time zones, it is still highly dependent on the country, and is subject to change.