Main article: History of longitude The Shepherd Gate Clock at the gates of the Royal Observatory, Greenwich is permanently kept on Greenwich Mean Time.Īs the United Kingdom developed into an advanced maritime nation, British mariners kept at least one chronometer on GMT to calculate their longitude from the Greenwich meridian, which was considered to have longitude zero degrees, by a convention adopted in the International Meridian Conference of 1884. The term GMT is especially used by United Kingdom bodies, such as the BBC World Service, the Royal Navy, and the Met Office and others particularly in Arab countries, such as the Middle East Broadcasting Centre and OSN. The term GMT should thus not be used for purposes that require precision. For navigation, it is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude) but this meaning can differ from UTC by up to 0.9 s. Today, Universal Time usually refers to Coordinated Universal Time (UTC) or UT1 English speakers often use GMT as a synonym for UTC. To avoid confusion, the name Universal Time was introduced in 1928 to denote GMT as counted from midnight. Originally, astronomers considered a GMT day to start at noon, while for almost everyone else it started at midnight. Noon GMT is the annual average (the arithmetic mean) moment of this event, which accounts for the word 'mean' in 'Greenwich Mean Time'. This event may occur up to 16 minutes before or after noon GMT, a discrepancy described by the equation of time. īecause of Earth's uneven angular velocity in its elliptical orbit and its axial tilt, noon (12:00:00) GMT is rarely the exact moment the Sun crosses the Greenwich Meridian and reaches its highest point in the sky there. The term GMT is also used as one of the names for the time zone UTC+00:00 and, in UK law, is the basis for civil time in the United Kingdom.

At different times in the past, it has been calculated in different ways, including being calculated from noon as a consequence, it cannot be used to specify a particular time unless a context is given. Greenwich Mean Time ( GMT) is the local mean time at the Royal Observatory in Greenwich, London, counted from midnight. Indiana introduced DST in 2006.A The islands of Cape Verde are to the west of the African mainland.ī Mauritius and the Seychelles are to the east and north-east of Madagascar respectively. Most of Arizona and Hawaii don't use DST. In addition the uninhabited atolls of Baker Island (AoE) and Wake Island (WAKT) add to the time zone count, making 11 the total number of time zones in the US.Īlmost all states in the US use Daylight Saving Time (DST). There are 9 official time zones according to the law. The time zones in the law are defined by their offset from Coordinated Universal Time (UTC). Code, Title 15, Chapter 6, Subchapter IX - Standard Time. Time zones in the USA are defined in the U.S. Since 1967, the US Department of Transportation (DOT) has been responsible for governing time zones in the country. The US was divided into 4 standard time zones on November 18, 1883, and jurisdiction for the zones was given to the Interstate Commerce Commission (ICC). Time Zones Being Used in Dependencies of United States Offset They will become active again after the next clock change as Daylight Saving Time begins or ends. The above time zones are used during other parts of the year. Time Zones Not Currently Being Observed in United States Offset

Note: Local time in these time zones changes when Daylight Saving Time begins and ends. In practice, this means that the local time in these time zones changes when DST begins and ends. For example, Eastern Time (ET) refers to Eastern Standard Time (EST) or Eastern Daylight Time (EDT), depending on which is currently in use. The time zones in the contiguous US are often referred to by their generic name, without making a difference between standard time and Daylight Saving Time designations. Generalized Time Zones in United States Time Zone Abbreviation & Name Since Howland Island and Baker Island constitute the world's westernmost landmasses in relation to the International Date Line, making them the last places on Earth where any date exists, they are sometimes assigned a theoretical 12th time zone called Anywhere on Earth (AoE). As neither Hawaii nor the 5 dependencies use Daylight Saving Time (DST), there are only 6 corresponding DST time zones.

In addition, Alaska, Hawaii, and 5 US dependencies all have their own time zones. The contiguous US has 4 standard time zones. However, adding the time zones of 2 uninhabited US territories, Howland Island and Baker Island, brings the total count to 11 time zones. There are 9 time zones by law in the USA and its dependencies. Time Zones Currently Being Used in United States Offset Business Date to Date (exclude holidays).

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