Universal Time

Universal Time (UT) is a time standard based on Earth's rotation. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, England. In fact, the expression "Universal Time" is ambiguous (when accuracy of better than a few seconds is required), as there are several versions of it, the most commonly used being Coordinated Universal Time (UTC) and UT1 (see § Versions).[1] All of these versions of UT, except for UTC, are based on Earth's rotation relative to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time. UTC is based on International Atomic Time, with leap seconds added to keep it within 0.9 second of UT1.[a]

Universal Time and standard time

Prior to the introduction of standard time, each municipality throughout the clock-using world set its official clock, if it had one, according to the local position of the Sun (see solar time). This served adequately until the introduction of rail travel in Britain, which made it possible to travel fast enough over long distances to require continuous re-setting of timepieces as a train progressed in its daily run through several towns. Greenwich Mean Time, where all clocks in Britain were set to the same time, was established to solve this problem. Chronometers or telegraphy were used to synchronize these clocks.[2]

World Time Zones Map
Standard time zones of the world since 2016. The number at the bottom of each timezone specifies the number of hours to add to UTC to convert it to the local time.

Standard time, as originally proposed by Scottish-Canadian Sir Sandford Fleming in 1879, divided the world into twenty-four time zones, each one covering 15 degrees of longitude. All clocks within each zone would be set to the same time as the others, but differed by one hour from those in the neighboring zones. The local time at the Royal Observatory in Greenwich was announced as the recommended base reference for world time on 22 October 1884 at the end of the International Meridian Conference.[b][3] This location was chosen because by 1884 two-thirds of all nautical charts and maps already used it as their prime meridian.[4] The conference did not adopt Fleming's time zones because they were outside the purpose for which it was called, which was to choose a basis for universal time (as well as a prime meridian).

During the period between 1848 and 1972, all of the major countries adopted time zones based on the Greenwich meridian.[5]

In 1935, the term Universal Time was recommended by the International Astronomical Union as a more precise term than Greenwich Mean Time, because GMT could refer to either an astronomical day starting at noon or a civil day starting at midnight.[6] The term Greenwich Mean Time persists, however, in common usage to this day in reference to civil timekeeping.


Based on the rotation of the Earth, time can be measured by observing celestial bodies crossing the meridian every day. Astronomers found that it was more accurate to establish time by observing stars as they crossed a meridian rather than by observing the position of the Sun in the sky. Nowadays, UT in relation to International Atomic Time (TAI) is determined by Very Long Baseline Interferometry (VLBI) observations of distant quasars, a method which can determine UT1 to within 15 microseconds or better.[7][8]

Universal Dial Plate or Times of all Nations, 1854
An 1853 "Universal Dial Plate" showing the relative times of "all nations" before the adoption of universal time

The rotation of the Earth and UT are monitored by the International Earth Rotation and Reference Systems Service (IERS). The International Astronomical Union also is involved in setting standards, but the final arbiter of broadcast standards is the International Telecommunication Union or ITU.[9]

The rotation of the Earth is somewhat irregular, and is very gradually slowing due to tidal acceleration. Furthermore, the length of the second was determined from observations of the Moon between 1750 and 1890. All of these factors cause the mean solar day, on the average, to be slightly longer than the nominal 86,400 SI seconds, the traditional number of seconds per day. As UT is slightly irregular in its rate, astronomers introduced Ephemeris Time, which has since been replaced by Terrestrial Time (TT). Because Universal Time is synchronous with night and day, and more precise atomic-frequency standards drift away from this, however, UT is still used to produce a correction (called a leap second) to atomic time, in order to obtain a broadcast form of civil time that carries atomic frequency. Thus, civil broadcast standards for time and frequency usually follow International Atomic Time closely, but occasionally step (or "leap") in order to prevent them from drifting too far from mean solar time.

Barycentric Dynamical Time (TDB), a form of atomic time, is now used in the construction of the ephemerides of the planets and other solar system objects, for two main reasons.[10] First, these ephemerides are tied to optical and radar observations of planetary motion, and the TDB time scale is fitted so that Newton's laws of motion, with corrections for general relativity, are followed. Next, the time scales based on Earth's rotation are not uniform and therefore, are not suitable for predicting the motion of bodies in our solar system.


There are several versions of Universal Time:

  • UT0 is Universal Time determined at an observatory by observing the diurnal motion of stars or extragalactic radio sources, and also from ranging observations of the Moon and artificial Earth satellites. The location of the observatory is considered to have fixed coordinates in a terrestrial reference frame (such as the International Terrestrial Reference Frame) but the position of the rotational axis of the Earth wanders over the surface of the Earth; this is known as polar motion. UT0 does not contain any correction for polar motion. The difference between UT0 and UT1 is on the order of a few tens of milliseconds. The designation UT0 is no longer in common use.[11]
  • UT1 is the principal form of Universal Time. While conceptually it is mean solar time at 0° longitude, precise measurements of the Sun are difficult. Hence, it is computed from observations of distant quasars using long baseline interferometry, laser ranging of the Moon and artificial satellites, as well as the determination of GPS satellite orbits. UT1 is the same everywhere on Earth, and is proportional to the rotation angle of the Earth with respect to distant quasars, specifically, the International Celestial Reference Frame (ICRF), neglecting some small adjustments. The observations allow the determination of a measure of the Earth's angle with respect to the ICRF, called the Earth Rotation Angle (ERA, which serves as a modern replacement for Greenwich Mean Sidereal Time). UT1 is required to follow the relationship
ERA = 2π(0.7790572732640 + 1.00273781191135448Tu) radians
where Tu = (Julian UT1 date - 2451545.0)[12]
  • UT1R is a smoothed version of UT1, filtering out periodic variations due to tides. It includes 62 smoothing terms, with periods ranging from 5.6 days to 18.6 years.[13]
  • UT2 is a smoothed version of UT1, filtering out periodic seasonal variations. It is mostly of historic interest and rarely used anymore. It is defined by
where t is the time as fraction of the Besselian year.[14]
  • UTC (Coordinated Universal Time) is an atomic timescale that approximates UT1. It is the international standard on which civil time is based. It ticks SI seconds, in step with TAI. It usually has 86,400 SI seconds per day but is kept within 0.9 seconds of UT1 by the introduction of occasional intercalary leap seconds. As of 2016, these leaps have always been positive (the days which contained a leap second were 86,401 seconds long). Whenever a level of accuracy better than one second is not required, UTC can be used as an approximation of UT1. The difference between UT1 and UTC is known as DUT1.[15]

Adoption in various countries

The table shows the dates of adoption of time zones based on the Greenwich meridian, including half-hour zones.

Year Countries [16]
1848 Great Britain[c]
1880 Ireland (entire island)
1883 Canada, USA[d]
1884 Serbia
1888 Japan
1892 Belgium, the Netherlands,[e] S. Africa[f]
1893 Italy, Germany, Austria-Hungary (railways)
1894 Bulgaria, Denmark, Norway, Switzerland, Romania, Turkey (railways)
1895 Australia, New Zealand, Natal
1896 Formosa (Taiwan)
1899 Puerto Rico, Philippines
1900 Sweden, Egypt, Alaska
1901 Spain
1902 Mozambique, Rhodesia
1903 Ts'intao, Tientsin
1904 China Coast, Korea, Manchuria, N. Borneo
1905 Chile
1906 India (except Calcutta), Ceylon (Sri Lanka), Seychelles
1907 Mauritius, Chagos
1908 Faroe Is., Iceland
1911 France, Algeria, Tunis, many French overseas possessions, British West Indies
1912 Portugal and overseas possessions, other French possessions, Samoa, Hawaii, Midway and Guam, Timor, Bismarck Arch., Jamaica, Bahamas Is.
1913 British Honduras, Dahomey
1914 Albania, Brazil, Colombia
1916 Greece, Poland, Turkey
Year Countries
1917 Iraq, Palestine
1918 Guatemala, Panama, Gambia, Gold Coast
1919 Latvia, Nigeria
1920 Argentine, Uruguay, Burma, Siam
1921 Finland, Estonia, Costa Rica
1922 Mexico
1924 Java, USSR
1925 Cuba
1928 China Inland
1930 Bermuda
1931 Paraguay
1932 Barbados, Bolivia, Dutch East Indies
1934 Nicaragua, E. Niger
By 1936 Labrador, Norfolk I.
By 1937 Cayman Is., Curaçao, Ecuador, Newfoundland
By 1939 Fernando Po, Persia
By 1940 Lord Howe I.
1940 The Netherlands
By 1948 Aden, Ascension I., Bahrein, British Somaliland, Calcutta, Dutch Guiana, Kenya, Federated Malay States, Oman, Straits Settlements, St. Helena, Uganda, Zanzibar
By 1953 Raratonga, South Georgia
By 1954 Cook Is.
By 1959 Maldive I. Republic
By 1961 Friendly Is., Tonga Is.
By 1962 Saudi Arabia
By 1964 Niue Is.
1972 Liberia

Apart from Nepal Standard Time (UTC+05:45), the Chatham Standard Time Zone (UTC+12:45) used in New Zealand's Chatham Islands[17] and the officially unsanctioned Central Western Time Zone (UTC+8:45) used in Eucla, Western Australia and surrounding areas, all timezones in use are defined by an offset from UTC that is a multiple of half an hour, and in most cases a multiple of an hour.

See also


  1. ^ The Earth's solar day is not constant.
  2. ^ voting took place on 13 October
  3. ^ legal in 1880
  4. ^ legal in 1918 (Standard Time Act)
  5. ^ Legal time reverted to Amsterdam time 1909; to Central European Time 1940,
  6. ^ except Natal
  1. ^ Guinot 2011, p. S181.
  2. ^ Howse 1997, ch. 4.
  3. ^ Howse 1997, pp. 12, 137.
  4. ^ Howse 1997, ch. 5.
  5. ^ Howse 1997, ch. 6.
  6. ^ McCarthy & Seidelmann 2009, p. 14.
  7. ^ McCarthy & Seidelmann 2009, pp. 68–9.
  8. ^ Urban & Seidelmann 2013, p. 175.
  9. ^ McCarthy & Seidelmann 2009, Ch. 18.
  10. ^ Urban & Seidelmann 2013, p. 7. Strictly speaking, a major producer of ephemerides, the Jet Propulsion Laboratory, uses a time scale they derive, Teph, which is functionally equivalent to TDB.
  11. ^ Urban & Seidelmann 2013, p. 81.
  12. ^ McCarthy & Seidelmann 2009, pp. 15–17, 62–64, 68–69, 76.
  13. ^ IERS n.d.
  14. ^ Date and Time Definitions n.d.
  15. ^ McCarthy & Seidelmann 2009, Ch. 14.
  16. ^ Howse 1980, pp. 154–5. Names have not been updated.
  17. ^ HM Nautical Almanac Office 2015.


  • "Date and Time Definitions". United States Naval Observatory. Retrieved 3 March 2013.
  • "Earth Rotation Variations Due to Zonal Tides". Paris: Earth Orientation Center. Retrieved 2 October 2011.
  • Galison, Peter (2003). Einstein's clocks, Poincaré's maps: Empires of time. New York: W.W. Norton & Co. ISBN 0-393-02001-0. Discusses the history of time standardization.
  • Guinot, Bernard (July 2011). "Solar time, legal time, time in use". Metrologia. 48 (4): S181–S185. Bibcode:2011Metro..48S.181G. doi:10.1088/0026-1394/48/4/S08.
  • HM Nautical Almanac Office (April 2015). "World Time Zone Map".
  • Howse, Derek (1980). Greenwich Time and the discovery of the longitude. Oxford Univ Press. pp. 154–5.. Names have not been updated.
  • Howse, Derek (1997). Greenwich Time and the Longitude. Phillip Wilson. ISBN 0-85667-468-0.
  • McCarthy, Dennis D. (July 1991). "Astronomical Time" (PDF). Proceedings of the IEEE. 79 (7): 915–920. doi:10.1109/5.84967.
  • McCarthy, Dennis; Seidelmann, P. Kenneth (2009). TIME—From Earth Rotation to Atomic Physics. Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA. ISBN 978-3-527-40780-4.
  • O'Malley, Michael (1996). Keeping watch: A history of American time. Washington DC: Smithsonian. ISBN 1-56098-672-7.
  • Seidelmann, P. Kenneth (1992). Explanatory supplement to the Astronomical Almanac. Mill Valley, California: University Science Books. ISBN 0-935702-68-7.
  • Urban, Sean; Seidelmann, P. Kenneth, eds. (2013). Explanatory Supplement to the Astronomical Almanac (3rd ed.). Mill Valley, California: University Science Books.
  • "UT1R". International Earth Rotation and Reference System Service. Retrieved 6 March 2013.
  • "What is TT?". Naval Oceanography Portal. United States Naval Observatory. Retrieved 3 March 2013.

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".

External links

  • Time Lord by Clark Blaise: a biography of Sanford Fleming and the idea of standard time
Coordinated Universal Time

Coordinated Universal Time (abbreviated to UTC) is the primary time standard by which the world regulates clocks and time. It is within about 1 second of mean solar time at 0° longitude, and is not adjusted for daylight saving time. In some countries where English is spoken, the term Greenwich Mean Time (GMT) is often used as a synonym for UTC and predates UTC by nearly 300 years.The first Coordinated Universal Time was informally adopted on 1 January 1960 and was first officially adopted as CCIR Recommendation 374, Standard-Frequency and Time-Signal Emissions, in 1963, but the official abbreviation of UTC and the official English name of Coordinated Universal Time (along with the French equivalent) were not adopted until 1967.The system has been adjusted several times, including a brief period where time coordination radio signals broadcast both UTC and "Stepped Atomic Time (SAT)" before a new UTC was adopted in 1970 and implemented in 1972. This change also adopted leap seconds to simplify future adjustments. This CCIR Recommendation 460 "stated that (a) carrier frequencies and time intervals should be maintained constant and should correspond to the definition of the SI second; (b) step adjustments, when necessary, should be exactly 1 s to maintain approximate agreement with Universal Time (UT); and (c) standard signals should contain information on the difference between UTC and UT."A number of proposals have been made to replace UTC with a new system that would eliminate leap seconds. A decision whether to remove them altogether has been deferred until 2023.The current version of UTC is defined by International Telecommunications Union Recommendation (ITU-R TF.460-6), Standard-frequency and time-signal emissions, and is based on International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the slowing of the Earth's rotation. Leap seconds are inserted as necessary to keep UTC within 0.9 seconds of the UT1 variant of universal time. See the "Current number of leap seconds" section for the number of leap seconds inserted to date.

Eastern European Summer Time

Eastern European Summer Time (EEST) is one of the names of UTC+03:00 time zone, 3 hours ahead of Coordinated Universal Time. It is used as a summer daylight saving time in some European and Middle Eastern countries, which makes it the same as Arabia Standard Time, East Africa Time and Moscow Time. During the winter periods, Eastern European Time (UTC+02:00) is used.

Since 1996 European Summer Time has been observed from the last Sunday in March to the last Sunday in October; previously the rules were not uniform across the European Union.

Eastern European Time

Eastern European Time (EET) is one of the names of UTC+02:00 time zone, 2 hours ahead of Coordinated Universal Time. The zone uses daylight saving time, so that it uses UTC+03:00 during the summer.

A number of African countries use UTC+02:00 all year long, where it is called Central Africa Time (CAT), although Egypt and Libya also use the term Eastern European Time.

Eastern Time Zone

The Eastern Time Zone (ET) is a time zone encompassing part or all of 22 states in the eastern part of the contiguous United States, parts of eastern Canada, the state of Quintana Roo in Mexico, Panama in Central America, and the Caribbean Islands, along with certain countries and parts of countries in South America. Places that use Eastern Standard Time (EST) when observing standard time (autumn/winter) are 5 hours behind Coordinated Universal Time (UTC−05:00).

Eastern Daylight Time (EDT), when observing daylight saving time (spring/summer), is 4 hours behind Coordinated Universal Time (UTC−04:00).

In the northern parts of the time zone, on the second Sunday in March, at 2:00 a.m. EST, clocks are advanced to 3:00 a.m. EDT leaving a one-hour "gap". On the first Sunday in November, at 2:00 a.m. EDT, clocks are moved back to 1:00 a.m. EST, thus "duplicating" one hour. Southern parts of the zone (Panama and the Caribbean) do not observe daylight saving time.

Greenwich Mean Time

Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, reckoned 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 precise time unless a context is given.

English speakers often use GMT as a synonym for Coordinated Universal Time (UTC). 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 not thus be used for technical purposes.Because of Earth's uneven speed 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. This event may occur up to 16 minutes before or after noon GMT, a discrepancy calculated by the equation of time. Noon GMT is the annual average (i.e. "mean") moment of this event, which accounts for the word "mean" in "Greenwich Mean Time".

Originally, astronomers considered a GMT day to start at noon, 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. Astronomers preferred the old convention to simplify their observational data, so that each night was logged under a single calendar date. Today Universal Time usually refers to UTC or UT1.The term "GMT" is especially used by bodies connected with the United Kingdom, such as the BBC World Service, the Royal Navy, the Met Office and others particularly in Arab countries, such as the Middle East Broadcasting Centre and OSN. It is a term commonly used in the United Kingdom and countries of the Commonwealth, including Australia, New Zealand, South Africa, India, Pakistan, Bangladesh and Malaysia; and in many other countries of the Eastern Hemisphere.

Julian day

Julian day is the continuous count of days since the beginning of the Julian Period and is used primarily by astronomers, and in software for easily calculating elapsed days between two events (e.g. food production date and sell by date).

The Julian Day Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Universal time, with Julian day number 0 assigned to the day starting at noon on Monday, January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic Gregorian calendar), a date at which three multi-year cycles started (which are: Indiction, Solar, and Lunar cycles) and which preceded any dates in recorded history. For example, the Julian day number for the day starting at 12:00 UT on January 1, 2000, was 2 451 545.The Julian date (JD) of any instant is the Julian day number plus the fraction of a day since the preceding noon in Universal Time. Julian dates are expressed as a Julian day number with a decimal fraction added. For example, the Julian Date for 00:30:00.0 UT January 1, 2013, is 2 456 293.520 833.The Julian Period is a chronological interval of 7980 years; year 1 of the Julian Period was 4713 BC. It has been used by historians since its introduction in 1583 to convert between different calendars. The Julian calendar year 2019 is year 6732 of the current Julian Period. The next Julian Period begins in the year AD 3268.

Nepal Standard Time

Nepal Standard Time (NPT) is the time zone for Nepal. With a time offset from Coordinated Universal Time (UTC) of UTC+05:45 all over Nepal, it is one of only three time zones with a 45-minute offset from UTC. (The others are Chatham Island Standard Time, with an offset of UTC+12:45, and the unofficial Australian Central Western Time, with an offset of UTC+08:45.)NPT is an approximation of Kathmandu mean time, which is 5:41:16 ahead of UTC. The standard meridian passes through the peak of Gaurishankar mountain about 100 km east of Kathmandu.Nepal used local solar time until 1920, in Kathmandu UTC+5:41:16. In 1920, Nepal adopted Indian Standard Time, UTC+05:30. In 1986 Nepal advanced their clocks by 15 minutes, giving them a time zone of UTC+05:45.

Pakistan Standard Time

Pakistan Standard Time (Urdu: پاکستان معیاری وقت‎, abbreviated as PST or sometimes PKT) is UTC+05:00 hours ahead of Coordinated Universal Time. The time zone is in use during standard time in Asia.

Time in Peru

Peru Time (PET) is the official time in Peru. It is always 5 hours behind Coordinated Universal Time (UTC−05:00). Peru has only one time zone and does not observe daylight saving time. During the winter (summer in the Northern Hemisphere), Peruvian Time is the same as North American Central Time, while during the summer (winter in the Northern Hemisphere) it is akin to Eastern Time.

UTC offset

The UTC offset is the difference in hours and minutes from Coordinated Universal Time (UTC) for a particular place and date. It is generally shown in the format ±[hh]:[mm], ±[hh][mm], or ±[hh]. So if the time being described is one hour ahead of UTC (such as the time in Berlin during the winter), the UTC offset would be "+01:00", "+0100", or simply "+01".

Every inhabited place in the world has a UTC offset that is a multiple of 15 minutes, and the majority of offsets (as well as all nautical time zones) are measured in whole hours.

UTC is the equivalent to GMT.

West Africa Time

West Africa Time, or WAT, is a time zone used in west-central Africa; with countries west of Benin instead using Greenwich Mean Time (GMT; equivalent to UTC with no offset). West Africa Time is one hour ahead of Coordinated Universal Time (UTC+01), which makes it the same as Central European Time (CET) during winter, or Western European Summer Time (WEST) and British Summer Time (BST) during the summer.

As most of this time zone is in the tropical region, there is little change in day length throughout the year, so daylight saving time is not observed.

West Africa Time is used by the following countries:

Algeria (as Central European Time)





Central African Republic

Democratic Republic of the Congo (western side only)

Equatorial Guinea


Morocco (as Central European Time)



Republic of the Congo

Tunisia (as Central European Time)

Western Sahara (as Central European Time)


In precise timekeeping, ΔT (Delta T, delta-T, deltaT, or DT) is a measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of atomic time. Formally it is the time difference obtained by subtracting Universal Time (UT, defined by the Earth's rotation) from Terrestrial Time (TT, independent of the Earth's rotation): ΔT = TT − UT. The value of ΔT for the start of 1902 is approximately zero; for 2002 it is about 64 seconds. So the Earth's rotations over that century took about 64 seconds longer than would be required for days of atomic time.

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