![World Time Zones Map](https://upload.wikimedia.org/wikipedia/commons/8/88/World_Time_Zones_Map.png)
Coordinated Universal Time or UTC is the primary
time standard
A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters o ...
by which the world regulates clocks and time. It is within about one second of
mean solar time
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Two types of solar time are apparent solar time ( sundial ...
(such as
UT1
Universal Time (UT or UT1) 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 ...
) at 0° longitude (at the
IERS Reference Meridian
The IERS Reference Meridian (IRM), also called the International Reference Meridian, is the prime meridian (0° longitude) maintained by the International Earth Rotation and Reference Systems Service (IERS). It passes about 5.3 arcseconds east ...
as the currently used
prime meridian
A prime meridian is an arbitrary meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great ...
) and is not adjusted for
daylight saving time
Daylight saving time (DST), also referred to as daylight savings time or simply daylight time (United States, Canada, and Australia), and summer time (United Kingdom, European Union, and others), is the practice of advancing clocks (typicall ...
. It is effectively a successor to
Greenwich Mean 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; as a con ...
(GMT).
The coordination of time and frequency transmissions around the world began on 1 January 1960. UTC 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 during which the 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 second
A leap second is a one- second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observ ...
s 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."
The General Conference on Weights and Measures adopted a resolution to alter UTC with a new system that would eliminate leap seconds by 2035.
The current version of UTC is defined by
International Telecommunication Union
The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
Recommendation (ITU-R TF.460-6), ''Standard-frequency and time-signal emissions'', and is based on
International Atomic Time
International Atomic Time (abbreviated TAI, from its French name ) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. TAI is a weighted average of the time kept by over 450 ato ...
(TAI) with leap seconds added at irregular intervals to compensate for the accumulated difference between TAI and time measured by
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.
Etymology
The official abbreviation for Coordinated Universal Time is ''UTC''. This abbreviation comes as a result of the
International Telecommunication Union
The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
and the
International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
wanting to use the same abbreviation in all languages. English speakers originally proposed ''CUT'' (for "coordinated universal time"), while
French speakers proposed ''TUC'' (for ""). The compromise that emerged was ''UTC'', which conforms to the pattern for the abbreviations of the variants of Universal Time (UT0, UT1, UT2, UT1R, etc.).
Uses
Time zone
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 ...
s around the world are expressed using
positive or negative offsets from UTC, as in the
list of time zones by UTC offset.
The westernmost time zone uses
UTC−12, being twelve hours behind UTC; the easternmost time zone uses
UTC+14
UTC+14:00 is an identifier for a time offset from UTC of +14:00. This is the earliest time zone on Earth, meaning that areas in this zone are the first to see a new day, and therefore the first to celebrate a New Year. It is also referred to as t ...
, being fourteen hours ahead of UTC. In 1995, the island nation of
Kiribati
Kiribati (), officially the Republic of Kiribati ( gil, ibaberikiKiribati),[Kiribati]
''The Wor ...
moved those of its atolls in the
Line Islands from
UTC−10 to
UTC+14
UTC+14:00 is an identifier for a time offset from UTC of +14:00. This is the earliest time zone on Earth, meaning that areas in this zone are the first to see a new day, and therefore the first to celebrate a New Year. It is also referred to as t ...
so that Kiribati would all be on the same day.
UTC is used in many
Internet
The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...
and
World Wide Web
The World Wide Web (WWW), commonly known as the Web, is an information system enabling documents and other web resources to be accessed over the Internet.
Documents and downloadable media are made available to the network through web ...
standards. The
Network Time Protocol (NTP), designed to synchronise the clocks of computers over the Internet, transmits time information from the UTC system. If only milliseconds precision is needed, clients can obtain the current UTC from a number of official internet UTC servers. For sub-microsecond precision, clients can obtain the time from satellite signals.
UTC is also the time standard used in
aviation
Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft such as hot a ...
, e.g. for
flight plans and
air traffic control
Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airs ...
.
Weather forecasts
Weather forecasting is the application of science and technology to predict the conditions of the atmosphere for a given location and time. People have attempted to predict the weather informally for millennia and formally since the 19th centu ...
and maps all use UTC to avoid confusion about time zones and daylight saving time. The
International Space Station
The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA ( ...
also uses UTC as a time standard.
Amateur radio operators often schedule their radio contacts in UTC, because transmissions on some frequencies can be picked up in many time zones.
Mechanism
UTC divides time into days, hours, minutes and seconds. Days are conventionally identified using the
Gregorian calendar
The Gregorian calendar is the calendar used in most parts of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years dif ...
, but
Julian day numbers can also be used. Each day contains 24 hours and each hour contains 60 minutes. The number of seconds in a minute is usually 60, but with an occasional
leap second
A leap second is a one- second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observ ...
, it may be 61 or 59 instead. Thus, in the UTC time scale, the second and all smaller time units (millisecond, microsecond, etc.) are of constant duration, but the minute and all larger time units (hour, day, week, etc.) are of variable duration. Decisions to introduce a leap second are announced at least six months in advance in "Bulletin C" produced by the
International Earth Rotation and Reference Systems Service. The leap seconds cannot be predicted far in advance due to the unpredictable rate of the rotation of Earth.
Nearly all UTC days contain exactly 86,400
SI seconds with exactly 60 seconds in each minute. UTC is within about one second of
mean solar time
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Two types of solar time are apparent solar time ( sundial ...
at
0° longitude, so that, because the
mean solar day
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Two types of solar time are apparent solar time (sundial ti ...
is slightly longer than 86,400 SI seconds, occasionally the last minute of a UTC day is adjusted to have 61 seconds. The extra second is called a leap second. It accounts for the grand total of the extra length (about 2 milliseconds each) of all the mean solar days since the previous leap second. The last minute of a UTC day is permitted to contain 59 seconds to cover the remote possibility of the Earth rotating faster, but that has not yet been necessary. The irregular day lengths mean that fractional Julian days do not work properly with UTC.
Since 1972, UTC is calculated by subtracting the accumulated leap seconds from
International Atomic Time
International Atomic Time (abbreviated TAI, from its French name ) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. TAI is a weighted average of the time kept by over 450 ato ...
(TAI), which is a
coordinate time
In the theory of relativity, it is convenient to express results in terms of a spacetime coordinate system relative to an implied observer. In many (but not all) coordinate systems, an event is specified by one time coordinate and three spatial ...
scale tracking notional
proper time
In relativity, proper time (from Latin, meaning ''own time'') along a timelike world line is defined as the time as measured by a clock following that line. It is thus independent of coordinates, and is a Lorentz scalar. The proper time interval ...
on the rotating surface of the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
(the
geoid
The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended ...
). In order to maintain a close approximation to
UT1
Universal Time (UT or UT1) 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 ...
, UTC occasionally has
discontinuities where it changes from one linear function of TAI to another. These discontinuities take the form of leap seconds implemented by a UTC day of irregular length. Discontinuities in UTC have occurred only at the end of June or December, although there is provision for them to happen at the end of March and September as well as a second preference. The International Earth Rotation and Reference Systems Service (IERS) tracks and publishes the difference between UTC and Universal Time,
DUT1 = UT1 − UTC, and introduces discontinuities into UTC to keep DUT1 in the
interval (−0.9 s, +0.9 s).
As with TAI, UTC is only known with the highest precision in retrospect. Users who require an approximation in real time must obtain it from a time laboratory, which disseminates an approximation using techniques such as
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
or radio
time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.
Church bells or voices announcing hours of prayer gave way to automatically operated chimes on public clocks; however, a ...
s. Such approximations are designated UTC(''k''), where ''k'' is an abbreviation for the time laboratory. The time of events may be provisionally recorded against one of these approximations; later corrections may be applied using the
International Bureau of Weights and Measures
The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organisation, through which its 59 member-states act together on measurement standards in four areas: chemistry ...
(BIPM) monthly publication of tables of differences between canonical TAI/UTC and TAI(''k'')/UTC(''k'') as estimated in real time by participating laboratories. (See the article on
International Atomic Time
International Atomic Time (abbreviated TAI, from its French name ) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. TAI is a weighted average of the time kept by over 450 ato ...
for details.)
Because of
time dilation
In physics and relativity, time dilation is the difference in the elapsed time as measured by two clocks. It is either due to a relative velocity between them ( special relativistic "kinetic" time dilation) or to a difference in gravitational ...
, a standard clock not on the geoid, or in rapid motion, will not maintain synchronicity with UTC. Therefore,
telemetry
Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots ''tele'', "remote", an ...
from clocks with a known relation to the geoid is used to provide UTC when required, on locations such as those of spacecraft.
It is not possible to compute the exact
time interval
Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, to c ...
elapsed between two UTC
timestamp
A timestamp is a sequence of characters or encoded information identifying when a certain event occurred, usually giving date and time of day, sometimes accurate to a small fraction of a second. Timestamps do not have to be based on some absolut ...
s without consulting a table that shows how many leap seconds occurred during that interval. By extension, it is not possible to compute the precise duration of a time interval that ends in the future and may encompass an unknown number of leap seconds (for example, the number of TAI seconds between "now" and 2099-12-31 23:59:59). Therefore, many scientific applications that require precise measurement of long (multi-year) intervals use TAI instead. TAI is also commonly used by systems that cannot handle leap seconds.
GPS time always remains exactly 19 seconds behind TAI (neither system is affected by the leap seconds introduced in UTC).
Time zones
Time zones are usually defined as differing from UTC by an integer number of hours, although the laws of each jurisdiction would have to be consulted if sub-second accuracy was required. Several jurisdictions have established time zones that differ by an odd integer number of half-hours or quarter-hours from UT1 or UTC.
Current
civil time
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 dayligh ...
in a particular
time zone
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 ...
can be determined by adding or subtracting the number of hours and minutes specified by the
UTC offset
The UTC offset is the difference in hours and minutes between Coordinated Universal Time (UTC) and local solar time, at a particular place. This difference is expressed with respect to UTC and is generally shown in the format , , or . So if the tim ...
, which ranges from
UTC−12:00 in the west to
UTC+14:00 in the east (see
List of UTC time offsets
This is a list of the UTC time offsets, showing the difference in hours and minutes from Coordinated Universal Time (UTC), from the westernmost (−12:00) to the easternmost (+14:00). It includes countries and regions that observe them during st ...
).
The time zone using UTC is sometimes denoted
UTC±00:00 or by the letter ''Z''—a reference to the equivalent
nautical time zone (GMT), which has been denoted by a ''Z'' since about 1950. Time zones were identified by successive letters of the alphabet and the Greenwich time zone was marked by a ''Z'' as it was the point of origin. The letter also refers to the "zone description" of zero hours, which has been used since 1920 (see
time zone history). Since the
NATO phonetic alphabet
The (International) Radiotelephony Spelling Alphabet, commonly known as the NATO phonetic alphabet, is the most widely used set of clear code words for communicating the letters of the Roman alphabet, technically a ''radiotelephonic spellin ...
word for ''Z'' is "Zulu", UTC is sometimes known as "Zulu time". This is especially true in aviation, where "Zulu" is the universal standard. This ensures that all pilots, regardless of location, are using the same
24-hour clock, thus avoiding confusion when flying between time zones. See the
list of military time zones Military time zones are defined in the ACP 121(I) standard, which is used by the armed forces for Australia, Canada, New Zealand, the United Kingdom, the United States, and many other nations. The names are identical to the NATO phonetic alphabet. ...
for letters used in addition to ''Z'' in qualifying time zones other than Greenwich.
On electronic devices which only allow the time zone to be configured using maps or city names, UTC can be selected indirectly by selecting cities such as
Accra in
Ghana
Ghana (; tw, Gaana, ee, Gana), officially the Republic of Ghana, is a country in West Africa. It abuts the Gulf of Guinea and the Atlantic Ocean to the south, sharing borders with Ivory Coast in the west, Burkina Faso in the north, and To ...
or
Reykjavík
Reykjavík ( ; ) is the capital and largest city of Iceland. It is located in southwestern Iceland, on the southern shore of Faxaflói bay. Its latitude is 64°08' N, making it the world's northernmost capital of a sovereign state. With a po ...
in
Iceland
Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...
as they are always on UTC and do not currently use
Daylight Saving Time
Daylight saving time (DST), also referred to as daylight savings time or simply daylight time (United States, Canada, and Australia), and summer time (United Kingdom, European Union, and others), is the practice of advancing clocks (typicall ...
(which
Greenwich
Greenwich ( , ,) is a town in south-east London, England, within the ceremonial county of Greater London. It is situated east-southeast of Charing Cross.
Greenwich is notable for its maritime history and for giving its name to the Greenwich ...
and
London
London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...
do, and so could be a source of error).
Daylight saving time
UTC does not change with a change of seasons, but
local time
Local time is the time observed in a specific locality. There is no canonical definition. Originally it was mean solar time, but since the introduction of time zones it is generally the time as determined by the time zone in effect, with daylight s ...
or civil time may change if a time zone jurisdiction observes daylight saving time (summer time). For example, local time on the east coast of the United States is five hours behind UTC during winter, but four hours behind while daylight saving is observed there.
History
In 1928, the term Universal Time (UT) was introduced by the International Astronomical Union to refer to GMT, with the day starting at midnight. Until the 1950s, broadcast
time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.
Church bells or voices announcing hours of prayer gave way to automatically operated chimes on public clocks; however, a ...
s were based on UT, and hence on the rotation of the Earth.
In 1955, the
caesium atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwe ...
was invented. This provided a form of timekeeping that was both more stable and more convenient than astronomical observations. In 1956, the U.S.
National Bureau of Standards
The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sci ...
and
U.S. Naval Observatory started to develop atomic frequency time scales; by 1959, these time scales were used in generating the
WWV time signals, named for the shortwave radio station that broadcasts them. In 1960, the U.S. Naval Observatory, the Royal Greenwich Observatory, and the
UK National Physical Laboratory coordinated their radio broadcasts so that time steps and frequency changes were coordinated, and the resulting time scale was informally referred to as "Coordinated Universal Time".
In a controversial decision, the frequency of the signals was initially set to match the rate of UT, but then kept at the same frequency by the use of atomic clocks and deliberately allowed to drift away from UT. When the divergence grew significantly, the signal was phase shifted (stepped) by 20
ms to bring it back into agreement with UT. Twenty-nine such steps were used before 1960.
In 1958, data was published linking the frequency for the
caesium transition, newly established, with the
ephemeris second. The ephemeris second is a unit in the system of time that, when used as the independent variable in the laws of motion that govern the movement of the planets and moons in the solar system, enables the laws of motion to accurately predict the observed positions of solar system bodies. Within the limits of observable accuracy, ephemeris seconds are of constant length, as are atomic seconds. This publication allowed a value to be chosen for the length of the atomic second that would accord with the celestial laws of motion.
In 1961, the
Bureau International de l'Heure
The International Time Bureau (french: Bureau International de l'Heure, abbreviated BIH), seated at the Paris Observatory, was the international bureau responsible for combining different measurements of Universal Time.
The bureau also played an ...
began coordinating the UTC process internationally (but the name Coordinated Universal Time was not formally adopted by the International Astronomical Union until 1967). From then on, there were time steps every few months, and frequency changes at the end of each year. The jumps increased in size to 0.1 seconds. This UTC was intended to permit a very close approximation to UT2.
In 1967, the
SI second was redefined in terms of the frequency supplied by a caesium atomic clock. The length of second so defined was practically equal to the second of ephemeris time. This was the frequency that had been provisionally used in TAI since 1958. It was soon decided that having two types of second with different lengths, namely the UTC second and the SI second used in TAI, was a bad idea. It was thought better for time signals to maintain a consistent frequency, and that this frequency should match the SI second. Thus it would be necessary to rely on time steps alone to maintain the approximation of UT. This was tried experimentally in a service known as "Stepped Atomic Time" (SAT), which ticked at the same rate as TAI and used jumps of 0.2 seconds to stay synchronised with UT2.
There was also dissatisfaction with the frequent jumps in UTC (and SAT). In 1968,
Louis Essen, the inventor of the caesium atomic clock, and G. M. R. Winkler both independently proposed that steps should be of 1 second only. This system was eventually approved, along with the idea of maintaining the UTC second equal to the TAI second. At the end of 1971, there was a final irregular jump of exactly 0.107758 TAI seconds, making the total of all the small time steps and frequency shifts in UTC or TAI during 1958–1971 exactly ten seconds, so that was exactly, and a whole number of seconds thereafter. At the same time, the tick rate of UTC was changed to exactly match TAI. UTC also started to track UT1 rather than UT2. Some time signals started to broadcast the DUT1 correction (UT1 − UTC) for applications requiring a closer approximation of UT1 than UTC now provided.
Current number of leap seconds
The first leap second occurred on 30 June 1972. Since then, leap seconds have occurred on average about once every 19 months, always on 30 June or 31 December. , there have been 27 leap seconds in total, all positive, putting UTC 37 seconds behind TAI.
Rationale
![Leapsecond](https://upload.wikimedia.org/wikipedia/commons/f/fb/Leapsecond.ut1-utc.svg)
Earth's
rotational speed
Rotational frequency (also known as rotational speed or rate of rotation) of an object rotating around an axis is the frequency of rotation of the object. Its unit is revolution per minute (rpm), cycle per second (cps), etc.
The symbol fo ...
is very slowly decreasing because of
tidal deceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit away from ...
; this increases the length of the
mean solar day
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Two types of solar time are apparent solar time (sundial ti ...
. The length of the SI second was calibrated on the basis of the second of
ephemeris time and can now be seen to have a relationship with the mean solar day observed between 1750 and 1892, analysed by
Simon Newcomb
Simon Newcomb (March 12, 1835 – July 11, 1909) was a Canadian–American astronomer, applied mathematician, and autodidactic polymath. He served as Professor of Mathematics in the United States Navy and at Johns Hopkins University. Born in N ...
. As a result, the SI second is close to of a mean solar day in the mid‑19th century. In earlier centuries, the mean solar day was shorter than 86,400 SI seconds, and in more recent centuries it is longer than 86,400 seconds. Near the end of the 20th century, the length of the mean solar day (also known simply as "length of day" or "LOD") was approximately 86,400.0013 s. For this reason, UT is now "slower" than TAI by the difference (or "excess" LOD) of 1.3 ms/day.
The excess of the LOD over the nominal 86,400 s accumulates over time, causing the UTC day, initially synchronised with the mean sun, to become desynchronised and run ahead of it. Near the end of the 20th century, with the LOD at 1.3 ms above the nominal value, UTC ran faster than UT by 1.3 ms per day, getting a second ahead roughly every 800 days. Thus, leap seconds were inserted at approximately this interval, retarding UTC to keep it synchronised in the long term. The actual
rotational period
The rotation period of a celestial object (e.g., star, gas giant, planet, moon, asteroid) may refer to its sidereal rotation period, i.e. the time that the object takes to complete a single revolution around its axis of rotation relative to the b ...
varies on unpredictable factors such as
tectonic motion and has to be observed, rather than computed.
Just as adding a leap day every four years does not mean the year is getting longer by one day every four years, the insertion of a leap second every 800 days does not indicate that the mean solar day is getting longer by a second every 800 days. It will take about 50,000 years for a mean solar day to lengthen by one second (at a rate of 2 ms per century). This rate fluctuates within the range of 1.7–2.3 ms/cy. While the rate due to
tidal friction
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit away f ...
alone is about 2.3 ms/cy, the
uplift of Canada and
Scandinavia
Scandinavia; Sámi languages: /. ( ) is a subregion in Northern Europe, with strong historical, cultural, and linguistic ties between its constituent peoples. In English usage, ''Scandinavia'' most commonly refers to Denmark, Norway, and Swe ...
by several metres since the
last ice age has temporarily reduced this to 1.7 ms/cy over the last 2,700 years. The correct reason for leap seconds, then, is not the current difference between actual and nominal LOD, but rather the ''accumulation'' of this difference over a period of time: Near the end of the 20th century, this difference was about of a second per day; therefore, after about 800 days, it accumulated to 1 second (and a leap second was then added).
In the graph of
DUT1 above, the excess of LOD above the nominal 86,400 s corresponds to the downward slope of the graph between vertical segments. (The slope became shallower in the 1980s, 2000s and late 2010s to 2020s because of slight accelerations of Earth's rotation temporarily shortening the day.) Vertical position on the graph corresponds to the accumulation of this difference over time, and the vertical segments correspond to leap seconds introduced to match this accumulated difference. Leap seconds are timed to keep DUT1 within the vertical range depicted by the adjacent graph. The frequency of leap seconds therefore corresponds to the slope of the diagonal graph segments, and thus to the excess LOD. Time periods when the slope reverses direction (slopes upwards, not the vertical segments) are times when the excess LOD is negative, that is, when the LOD is below 86,400 s.
Future
As the Earth's rotation continues to slow, positive leap seconds will be required more frequently. The long-term
rate of change of LOD is approximately +1.7 ms per century. At the end of the 21st century, LOD will be roughly 86,400.004 s, requiring leap seconds every 250 days. Over several centuries, the frequency of leap seconds will become problematic. A change in the trend of the UT1 – UTC values was seen beginning around June 2019 in which instead of slowing down (with leap seconds to keep the difference between UT1 and UTC less than 0.9 seconds) the earth's rotation has sped up, causing this difference to increase. If the trend continues, a negative leap second may be required, which has not been used before. This may not be needed until 2025.
Some time in the 22nd century, two leap seconds will be required every year. The current practice of only allowing leap seconds in June and December will be insufficient to maintain a difference of less than 1 second, and it might be decided to introduce leap seconds in March and September. In the 25th century, four leap seconds are projected to be required every year, so the current quarterly options would be insufficient.
In April 2001, Rob Seaman of the
National Optical Astronomy Observatory proposed that leap seconds be allowed to be added monthly rather than twice yearly.
A resolution has been adopted by the General Conference on Weights and Measures to redefine UTC and abolish leap seconds, but keep the civil second constant and equal to the SI second, so that
sundials would slowly get further and further out of sync with civil time. The leap seconds will be eliminated by 2035. The resolution does not break the connection between UTC and UT1, but increases the maximum allowable difference. The details of what the maximum difference will be and how corrections will be implemented is left for future discussions.
[ This will result in a shift of the sun's movements relative to civil time, with the difference increasing quadratically with time (i.e., proportional to elapsed centuries squared). This is analogous to the shift of ]season
A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and ...
s relative to the yearly calendar that results from the calendar year not precisely matching the tropical year
A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time f ...
length. This would be a change in civil timekeeping, and would have a slow effect at first, but becoming drastic over several centuries. UTC (and TAI) would be more and more ahead of UT; it would coincide with local mean time along a meridian drifting eastward faster and faster. Thus, the time system will lose its fixed connection to the geographic coordinates based on the IERS meridian. The difference between UTC and UT would reach 0.5 hours after the year 2600 and 6.5 hours around 4600.
ITU‑R Study Group 7 and Working Party 7A were unable to reach consensus on whether to advance the proposal to the 2012 Radiocommunications Assembly; the chairman of Study Group 7 elected to advance the question to the 2012 Radiocommunications Assembly (20 January 2012), but consideration of the proposal was postponed by the ITU until the World Radio Conference in 2015. This conference, in turn, considered the question, but no permanent decision was reached; it only chose to engage in further study with the goal of reconsideration in 2023.
A proposed alternative to the leap second is the leap hour or leap minute, which requires changes only once every few centuries.
See also
* Coordinated Mars Time (MTC)
* Ephemeris time
* IERS Reference Meridian
The IERS Reference Meridian (IRM), also called the International Reference Meridian, is the prime meridian (0° longitude) maintained by the International Earth Rotation and Reference Systems Service (IERS). It passes about 5.3 arcseconds east ...
* ISO 8601
ISO 8601 is an international standard covering the worldwide exchange and communication of date and time-related data. It is maintained by the Geneva-based International Organization for Standardization (ISO) and was first published in 1988, w ...
* List of UTC timing centers
List of UTC timing centers is a list of over 70 recognized maintainers of atomic clocks around the world from which UTC is calculated. Below are links to some of the more notable time centers which contribute to the calculation of UTC. UTC is cal ...
* Terrestrial Time
* Universal Time
Universal Time (UT or UT1) 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 wit ...
* World Radiocommunication Conference
The World Radiocommunication Conference (WRC) is a conference organized by the ITU to review and, as necessary, revise the Radio Regulations, the international treaty governing the use of the radio-frequency spectrum as well as geostationary and ...
References
Citations
General and cited sources
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* See heading "NTP Timescale and Data Formats".
* Resolution no. 3 by Commissions 4 (Ephemerides/Ephémérides) and 31 (Time/L'Heure) (near the end of the document) "recommend that the following notations be used in all languages", UT0(i), UT1(i), UT2(i), UTC, UTC(i), UT, where (i) is institution "i".
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External links
''Current'' UTC time
* ttps://hpiers.obspm.fr/eop-pc/earthor/utc/TAI-UTC_tab.html International Earth Rotation Service; list of differences between TAI and UTC from 1961 to present
W3C Specification about UTC Date and Time
and , based on ISO 8601
ISO 8601 is an international standard covering the worldwide exchange and communication of date and time-related data. It is maintained by the Geneva-based International Organization for Standardization (ISO) and was first published in 1988, w ...
Standard of time definition: UTC, GPS, LORAN and TAI
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{{UTC time offsets
ITU-R recommendations
Time scales