What is gmt in time?

Greenwich Mean Time (GMT) is the mean solar 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;[1] 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,[2] in UK law, is the basis for civil time in the United Kingdom.[3][a]

English speakers often use GMT as a synonym for Coordinated Universal Time (UTC).[4] 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. The term GMT should thus not be used for purposes that require precision.[5]

Because 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[b] and reaches its highest point in the sky there. This event may occur up to 16 minutes before or after noon GMT, a discrepancy described by the equation of time. Noon GMT is the annual average (the arithmetic mean) moment of this event, which accounts for the word "mean" in "Greenwich Mean Time".[c]

Originally, astronomers considered a GMT day to start at noon,[d] while for almost everyone else it started at midnight. To avoid confusion, the name Universal Time was introduced to denote GMT as counted from midnight.[7] Today, Universal Time usually refers to UTC or UT1.[8]

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.

As 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. Synchronisation of the chronometer on GMT did not affect shipboard time, which was still solar time. But this practice, combined with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances based on observations at Greenwich, led to GMT being used worldwide as a standard time independent of location. Most time zones were based upon GMT, as an offset of a number of hours (and possibly half or quarter hours) "ahead of GMT" or "behind GMT".

Greenwich Mean Time was adopted across the island of Great Britain by the Railway Clearing House in 1847 and by almost all railway companies by the following year, from which the term "railway time" is derived. It was gradually adopted for other purposes, but a legal case in 1858 held "local mean time" to be the official time.[9] On 14 May 1880, a letter signed by "Clerk to Justices" appeared in The Times, stating that "Greenwich time is now kept almost throughout England, but it appears that Greenwich time is not legal time. For example, our polling booths were opened, say, at 8 13 and closed at 4 13 p.m."[10][11] This was changed later in 1880, when Greenwich Mean Time was legally adopted throughout the island of Great Britain. GMT was adopted in the Isle of Man in 1883, in Jersey in 1898 and in Guernsey in 1913. Ireland adopted GMT in 1916, supplanting Dublin Mean Time.[12] Hourly time signals from Greenwich Observatory were first broadcast on 5 February 1924, rendering the time ball at the observatory redundant.

The daily rotation of the Earth is irregular (see ΔT) and has a slowing trend; therefore atomic clocks constitute a much more stable timebase. On 1 January 1972, GMT as the international civil time standard was superseded by Coordinated Universal Time (UTC), maintained by an ensemble of atomic clocks around the world. Universal Time (UT), a term introduced in 1928, initially represented mean time at Greenwich determined in the traditional way to accord with the originally defined universal day; from 1 January 1956 (as decided by the International Astronomical Union in Dublin in 1955, at the initiative of William Markowitz) this "raw" form of UT was re-labelled UT0 and effectively superseded by refined forms UT1 (UT0 equalised for the effects of polar wandering)[13] and UT2 (UT1 further equalised for annual seasonal variations in Earth rotation rate).

Historically, GMT has been used with two different conventions for numbering hours. The long-standing astronomical convention, dating from the work of Ptolemy, was to refer to noon as zero hours (see Julian day). This contrasted with the civil convention of referring to midnight as zero hours dating from the Roman Empire. The latter convention was adopted on and after 1 January 1925 for astronomical purposes, resulting in a discontinuity of 12 hours, or half a day. The instant that was designated as "December 31.5 GMT" in 1924 almanacs became "January 1.0 GMT" in 1925 almanacs. The term Greenwich Mean Astronomical Time (GMAT) was introduced to unambiguously refer to the previous noon-based astronomical convention for GMT.[14] The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours.

Legally, the civil time used in the UK is called "Greenwich mean time" (without capitalisation), according to the Interpretation Act 1978, with an exception made for those periods when the Summer Time Act 1972 orders an hour's shift for daylight saving. The Interpretation Act 1978, section 9, provides that whenever an expression of time occurs in an Act, the time referred to shall (unless otherwise specifically stated) be held to be Greenwich mean time.[3] Under subsection 23, the same rule applies to deeds and other instruments.[12]

During the experiment of 1968 to 1971, when the British Isles did not revert to Greenwich Mean Time during the winter, the all-year British Summer Time was called British Standard Time (BST).

In the UK, UTC+00:00 is disseminated to the general public in winter and UTC+01:00 in summer.[7][15]

BBC radio stations broadcast the "six pips" of the Greenwich Time Signal. It is named from its original generation at the Royal Greenwich Observatory. If announced (such as near the start of summer time or of winter time), announcers on domestic channels declare the time as GMT or BST as appropriate. As the BBC World Service is broadcast to all time zones, the announcers use the term "Greenwich Mean Time" consistently throughout the year.

Several countries define their local time by reference to Greenwich Mean Time.[16][17] Some examples are:

Greenwich Mean Time is defined in law as standard time in the following countries and areas, which also advance their clocks one hour (GMT+1) in summer.

Greenwich Mean Time is used as standard time all year round in the following countries and areas:

The Royal Observatory Greenwich is the home of Greenwich Mean Time (GMT). But what is GMT and why is it so important?

Greenwich Mean Time is the yearly average (or ‘mean’) of the time each day when the Sun crosses the Prime Meridian at the Royal Observatory Greenwich.

Essentially, mean time is clock time rather than solar (astronomical) time.

Solar time varies throughout the year, as the time interval between the Sun crossing a set meridian line changes.

But each day measured by a clock has the same length, equal to the average (mean) length of a solar day. It’s a way of standardising and regularising time so we can all know exactly what time it is for our (or anyone’s) location.

Today GMT is reckoned from one midnight to the next.

Find out about the Prime Meridian

GMT stands for Greenwich Mean Time, the local clock time at Greenwich. From 1884 until 1972, GMT was the international standard of civil time. Though it has now been replaced by Coordinated Universal Time (UTC), GMT is still the legal time in Britain in the winter, used by the Met Office, Royal Navy and BBC World Service. Greenwich Mean Time is also the name of the time zone used by some countries in Africa and Western Europe, including in Iceland all year round.

It wasn’t until the invention of the pendulum clock in the 1650s that it was possible to work out the relationship between mean (clock) time and solar time.

John Flamsteed came up with the formula for converting solar time to mean time, and published a set of conversion tables in the early 1670s. Soon after, he was appointed as the first Astronomer Royal and moved into the new Royal Observatory in Greenwich.

Here he had the best pendulum clocks installed and set them to the local time. This was Greenwich Mean Time, or the average time when the Sun crossed the meridian at Greenwich. At first though, Greenwich time was only really important to astronomers.

In the 1700s, the fifth Astronomer Royal Nevil Maskelyne brought Greenwich Mean Time to a wider audience.

In 1767 Maskelyne introduced the Nautical Almanac as part of the great 18th century quest to determine longitude.

These were tables of ‘lunar distance’ data based on observations at Greenwich and using GMT as the time standard. This data enabled navigators to find their position at sea.

GMT was also crucial to the other great solution to the ‘longitude problem’, represented by John Harrison’s famous timekeepers.

British mariners started keeping at least one chronometer set to GMT. This meant they could calculate their longitude from the Greenwich meridian (longitude 0° by convention).

These two solutions would help pave the way for GMT to become the worldwide time standard a century later.

Find out about the longitude problem

Until the mid-19th century, almost every town kept its own local time, defined by the Sun. There were no national or international conventions which set how time should be measured.

This meant there was no standard timings for when the day would begin and end, or what length an hour might be. As well as Greenwich Mean Time for example, there was also Bristol Mean Time (10 minutes behind GMT) Cardiff Mean Time (13 minutes behind GMT).

However, the 1850s and 1860s saw the expansion of the railway and communications networks. This meant the need for an national time standard became imperative.

British railway companies started introducing a single standard time across their networks, designed to make their timetables less confusing. It was mostly Greenwich Mean Time that they used. GMT was ultimately adopted across Great Britain by the Railway Clearing House in December 1847. It officially became 'Railway Time'.

By the mid-1850s, almost all public clocks in Britain were set to Greenwich Mean Time and it finally became Britain’s legal standard time in 1880.

In 1884 the Greenwich Meridian was recommended as the Prime Meridian of the World.

There were two main reasons for this. The first was that the USA had already chosen Greenwich as the basis for its own national time zone system. The second was that in the late 19th century, 72% of the world's commerce depended on sea-charts which used Greenwich as the Prime Meridian.

The recommendation was based on the argument that naming Greenwich as Longitude 0º would be of advantage to the largest number of people.

As the reference for GMT, the Prime Meridian at Greenwich therefore became the centre of world time and the basis for the global system of time zones.

The Airy Transit Circle (telescope) became the telescope that would define the Prime Meridian of the World. Astronomer Royal George Biddell Airy designed it, and it is located at the Royal Observatory Greenwich.

It was recommended that the meridian line would indicate 0° longitude. Therefore this also became the start of the Universal Day. The meridian line is marked by the cross-hairs in the Airy Transit Circle eyepiece.

Find out more about the Airy Transit Circle

The Shepherd gate clock can be seen at the gates to the Royal Observatory. It was the first clock ever to show Greenwich Mean Time directly to the public. It is a 'slave' clock, connected to the Shepherd master clock which was installed at the Royal Observatory in 1852.

From that time until 1893, the Shepherd master clock was the heart of Britain's time system. Its time was sent by telegraph wires to London, Edinburgh, Glasgow, Dublin, Belfast and many other cities. By 1866, time signals were also sent from the clock to Harvard University in Cambridge, Massachusetts via the new transatlantic submarine cable.

In terms of the distribution of accurate time into everyday life, it is one of the most important clocks ever made.

The first thing you notice about the clock is that it has 24 hours on its face rather than the usual 12. That means at 12 noon the hour hand is pointing straight down rather than straight up.

Weather observations around the world (including surface, radar, and other observations) are always taken with respect to a standard time. By convention, the world's weather communities use a twenty four hour clock, similar to "military" time based on the 0° longitude meridian, also known as the Greenwich meridian.

Prior to 1972, this time was called Greenwich Mean Time (GMT) but is now referred to as Coordinated Universal Time or Universal Time Coordinated (UTC). It is a coordinated time scale, maintained by the Bureau International des Poids et Mesures (BIPM). It is also known as "Z time" or "Zulu Time".

To obtain your local time here in the United States, you need to subtract a certain number of hours from UTC depending on how many time zones you are away from Greenwich (England). The table below shows the standard difference from UTC time to local time.

The switch to daylight saving time does not affect UTC. It refers to time on the zero or Greenwich meridian, which is not adjusted to reflect changes either to or from Daylight Saving Time.

However, you need to know what happens during daylight saving time in the United States. In short, the local time is advanced one hour during daylight saving time. As an example, the Eastern Time zone difference from UTC is -4 hours during daylight saving time rather than -5 hours as it is during standard time.

Note:  Credit given to NWS Southern Region Headquarters for the information above.