Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory in Greenwich, London, counted from midnight. 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. 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.
A time zone is an area which observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it is convenient for areas in frequent communication to keep the same time.
Universal Time is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle with respect to the International Celestial Reference Frame (ICRF), called the Earth Rotation Angle. UT1 is the same everywhere on Earth. UT1 is required to follow the relationship
The Royal Observatory, Greenwich is an observatory situated on a hill in Greenwich Park in south east London, overlooking the River Thames to the north. It played a major role in the history of astronomy and navigation, and because the Prime Meridian passed through it, it gave its name to Greenwich Mean Time, the precursor to today's Coordinated Universal Time (UTC). The ROG has the IAU observatory code of 000, the first in the list. ROG, the National Maritime Museum, the Queen's House and the clipper ship Cutty Sark are collectively designated Royal Museums Greenwich.
The Royal Borough of Greenwich is a London borough in southeast Greater London, England. The London Borough of Greenwich was formed in 1965 by the London Government Act 1963. The new borough covered the former area of the Metropolitan Borough of Greenwich and part of the Metropolitan Borough of Woolwich to the east. The local council is Greenwich London Borough Council which meets in Woolwich Town Hall. The council's offices are also based in Woolwich, the main urban centre in the borough.
Greenwich is a town in south-east London, England, within the ceremonial county of Greater London. It is situated 5.5 miles (8.9 km) east-southeast of Charing Cross.
The International Meridian Conference was a conference held in October 1884 in Washington, D.C., in the United States, to determine a prime meridian for international use. The conference was held at the request of U.S. President Chester A. Arthur. The subject to discuss was the choice of "a meridian to be employed as a common zero of longitude and standard of time reckoning throughout the world". It resulted in the recommendation of the Greenwich Meridian as the international standard for zero degrees longitude.
In the United States, time is divided into nine standard time zones covering the states, territories and other US possessions, with most of the country observing daylight saving time (DST) for approximately the spring, summer, and fall months. The time zone boundaries and DST observance are regulated by the Department of Transportation, but no single map of those existed until the agency announced intentions to make one in September 2022. Official and highly precise timekeeping services (clocks) are provided by two federal agencies: the National Institute of Standards and Technology (NIST) ; and the United States Naval Observatory (USNO). The clocks run by these services are kept synchronized with each other as well as with those of other international timekeeping organizations.
Standard time is the synchronization of clocks within a geographical region to a single time standard, rather than a local mean time standard. Generally, standard time agrees with the local mean time at some meridian that passes through the region, often near the centre of the region. Historically, standard time was established during the 19th century to aid weather forecasting and train travel. Applied globally in the 20th century, the geographical regions became time zones. The standard time in each time zone has come to be defined as an offset from Universal Time. A further offset is applied for part of the year in regions with daylight saving time.
A time ball or timeball is a time-signalling device. It consists of a large, painted wooden or metal ball that is dropped at a predetermined time, principally to enable navigators aboard ships offshore to verify the setting of their marine chronometers. Accurate timekeeping is essential to the determination of longitude at sea.
In modern usage, civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally national standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972. Older systems use telescope observations.
The Dunsink Observatory is an astronomical observatory established in 1785 in the townland of Dunsink in the outskirts of the city of Dublin, Ireland.
During British Summer Time (BST), civil time in the United Kingdom is advanced one hour forward of Greenwich Mean Time (GMT), in effect changing the time zone from UTC±00:00 to UTC+01:00, so that mornings have one hour less daylight, and evenings one hour more.
Canada is divided into six time zones. Most areas of the country's provinces and territories operate on standard time from the first Sunday in November to the second Sunday in March and daylight saving time the rest of the year.
The United Kingdom uses Greenwich Mean Time or Western European Time (UTC) and British Summer Time or Western European Summer Time (UTC+01:00).
Malaysian Standard Time or Malaysian Time (MYT) is the standard time used in Malaysia. It is 8 hours ahead of Coordinated Universal Time (UTC). Malaysia does not observe daylight saving time.
UTC+00:00 is an identifier for a time offset from UTC of +00:00. This time zone is the basis of UTC and all other time zones are based on it. In ISO 8601, an example of the associated time would be written as 2023-01-01T12:12:34+00:00. It is also known by the following geographical or historical names:
Railway time was the standardised time arrangement first applied by the Great Western Railway in England in November 1840, the first recorded occasion when different local mean times were synchronised and a single standard time applied. The key goals behind introducing railway time were to overcome the confusion caused by having non-uniform local times in each town and station stop along the expanding railway network and to reduce the incidence of accidents and near misses, which were becoming more frequent as the number of train journeys increased.
Myanmar Standard Time, formerly Burma Standard Time (BST), is the standard time in Myanmar, 6.5 hours ahead of UTC. Myanmar Standard Time (MMT) is calculated on the basis of 97°30′E longitude. MMT is used all year round, as Myanmar does not observe daylight saving time.
Ireland uses Irish Standard Time in the summer months and Greenwich Mean Time in the winter period.
