These time zones encompass most of the contiguous United States. Likely the most well-known include Eastern, Central, Mountain, and Pacific Time Zones. In the U.S., there are a total of 9 time zones used. As a general rule of thumb, a change of 15° of longitude should result in a time difference of 1 hour. Many countries have more than one time zone.

The following map displays the standard time zones across the world. 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. Russia on the other hand, is divided into 11 time zones, though at one point this was reduced to 9 time zones. 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. Generally, a time change of 1 hour is required with each 15° change of longitude, but this does not necessarily always happen. 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. It wasn't until 1929 before most countries adopted hourly time zones. Time zones throughout the world vary, and used to vary even more than they currently do. 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. UTC offset can range from UTC-12:00 to UTC+14:00. 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 breaks time into days, hours, minutes, and seconds, where days are usually defined in terms of the Gregorian calendar. Most time zones that are on land are offset from UTC. 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. UTC, which is based on highly precise atomic clocks and the Earth's rotation, is the new standard of today. 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. 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.

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. They are often based on the boundaries of countries or lines of longitude. Related Time Calculator | Date CalculatorĪ time zone is a region on Earth that uses a uniform time.

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