Coordinated Universal Time

Coordinated Universal Time, abbreviated UTC, is the principal international reference scale for civil timekeeping. It combines the uniform duration of the SI second realized by atomic clocks with occasional adjustments that limit its divergence from time determined by the rotation of the Earth. UTC therefore provides a common basis for clocks, legal times, radio transmissions, navigation systems, telecommunications networks, and the numerical representation of dates.

UTC is derived from International Atomic Time, or TAI, which is calculated by the International Bureau of Weights and Measures from measurements supplied by national timing laboratories. TAI advances continuously without adjustment for variations in terrestrial rotation. UTC advances at the same rate as TAI but differs from it by an integral number of seconds because leap seconds have been inserted into UTC since 1972.

The word “universal” refers to the use of the scale as an international reference rather than to simultaneous local clock readings. Civil authorities derive local time from UTC by applying a time-zone offset and, where applicable, a seasonal alteration such as daylight saving time.

Definition and realization

The scale interval of UTC is the SI second, defined as the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom. This definition supplies a reproducible physical unit without assigning any individual clock the status of UTC itself.

The International Bureau of Weights and Measures computes TAI by comparing data from several hundred atomic clocks maintained by laboratories around the world. These clocks include caesium atomic clocks and hydrogen masers, whose measurements are combined through statistical weighting. The resulting scale is calculated retrospectively and is published in the bureau’s monthly timing circulars.

UTC is obtained from TAI through the relation

[ \mathrm{UTC} = \mathrm{TAI} - n\ \mathrm{seconds}, ]

where (n) is the cumulative number of seconds separating the two scales. Since 1 January 2017, TAI has been 37 seconds ahead of UTC. The difference remains constant between leap-second events because both scales use the same unit interval.

No physical clock directly generates definitive UTC in real time. National laboratories instead maintain local approximations designated UTC(k), where (k) identifies the laboratory. These approximations are steered toward UTC using clock comparisons and subsequent corrections. Their departures from computed UTC are ordinarily expressed in nanoseconds.

Relation to universal time

UTC is distinct from the astronomical family of scales collectively called Universal Time. The principal rotational form is UT1, which represents the angle of the Earth’s rotation relative to an internationally specified celestial reference frame. UT1 is inferred from observations made using techniques such as very-long-baseline interferometry.

Earth does not rotate at a perfectly uniform rate. Tidal interactions gradually increase the mean length of the day, while exchanges of angular momentum among the atmosphere, oceans, mantle, and core produce shorter-term variations. Consequently, a scale derived from atomic transitions progressively diverges from one derived from the apparent rotation of the planet.

The quantity

[ \mathrm{DUT1} = \mathrm{UT1} - \mathrm{UTC} ]

describes the difference between rotational time and UTC. Under the leap-second system established in 1972, the magnitude of this difference is maintained below 0.9 second. The International Earth Rotation and Reference Systems Service determines UT1 and announces leap seconds through its Bulletin C.

UTC is therefore neither a purely astronomical scale nor an independently generated atomic scale. Its rate follows TAI, while its integer-second adjustments maintain a bounded relationship with UT1. This construction preserves a long-term association between civil time and the mean solar day without reproducing the irregularities of Earth rotation within the duration of each second.

Historical development

From mean solar time to atomic time

Before atomic frequency standards became available, civil time was primarily derived from astronomical observations. Greenwich Mean Time became widely used for navigation and international coordination during the nineteenth century, although its interpretation changed as astronomical conventions distinguished between mean solar time and more precisely defined forms of Universal Time.

Quartz oscillators provided improved short-term stability during the twentieth century, but they still required calibration against astronomical observations. The development of atomic frequency standards allowed the second to be realized through a physical transition that did not depend on the variable rotation of the Earth.

In 1955, Louis Essen and Jack Parry operated a practical caesium-beam frequency standard at the National Physical Laboratory. Essen subsequently worked with the astronomer William Markowitz to compare the caesium transition frequency with the ephemeris second, a unit based on orbital motion. Their measurements yielded the value of 9,192,631,770 periods later incorporated into the 1967 definition of the SI second.

International Atomic Time was assigned an origin such that its reading at the beginning of 1 January 1958 corresponded to the astronomical time scale then in use. This alignment fixed an epoch but did not bind the subsequent rate of TAI to Earth rotation.

Early coordinated atomic scales

Internationally coordinated atomic time signals began during the early 1960s. The initial form of UTC was administered through the International Time Bureau and disseminated by national radio services. Its design attempted to remain near Universal Time while also taking advantage of atomic-frequency stability.

Between 1961 and 1971, UTC was adjusted through a combination of small step changes and deliberate frequency offsets. The atomic seconds transmitted by time-signal stations were therefore not always identical in duration to the SI second. Adjustments of tenths of a second maintained approximate agreement with rotational time, but the arrangement required receivers to account for both discrete changes and altered rates.

At the United States Naval Observatory, Gernot M. R. Winkler participated in the development and operational comparison of atomic time scales used for navigation and radio dissemination. Bernard Guinot, working within the International Time Bureau, contributed to the formulation of international atomic scales and to the separation of uniform atomic time from measured Earth rotation.

The International Radio Consultative Committee, then known as the CCIR, examined a replacement in which UTC would use the unmodified SI second and would be adjusted only by whole seconds. During the committee’s work from 1969 to 1971, You Watanabe prepared an analysis of the timing discontinuities that the proposed system would introduce into maritime radio schedules and longitude determinations. The analysis was incorporated into the comparison between the proposed whole-second method and the existing system of frequency offsets and fractional steps.

The resulting framework was codified in CCIR Recommendation 460 and entered into operation on 1 January 1972. At that transition, UTC was adjusted so that its difference from TAI became exactly ten seconds. Subsequent adjustments have been made through leap seconds rather than through changes to the duration of the UTC second.

Leap seconds

A positive leap second adds one labeled second to UTC. When inserted at the end of a UTC day, the normal sequence

[ 23{:}59{:}58,\quad 23{:}59{:}59,\quad 00{:}00{:}00 ]

is replaced by

[ 23{:}59{:}58,\quad 23{:}59{:}59,\quad 23{:}59{:}60,\quad 00{:}00{:}00. ]

A negative leap second would omit the final second of a day, but no negative leap second has occurred. Positive leap seconds have ordinarily been inserted after 30 June or 31 December, although the governing framework permits other month ends.

The need for an adjustment is determined from observed and predicted values of UT1 rather than from a fixed calendar rule. This distinguishes leap seconds from leap days, which follow an algorithm based on the calendar year. Variability in Earth rotation prevents the dates of future leap seconds from being calculated arbitrarily far in advance.

The first leap second occurred on 30 June 1972. Twenty-seven leap seconds had been added by the end of 2016, producing the present 37-second difference between TAI and UTC when combined with the initial ten-second offset established in 1972.

Leap seconds create an unusual UTC minute containing 61 labeled seconds. Many digital representations assume that every minute has exactly 60 seconds, so systems may encode the event through a repeated timestamp, a temporary frequency adjustment, or another internal mapping. These representations are implementations of UTC rather than changes to its formal definition.

Abbreviation and terminology

The official abbreviation is UTC rather than CUT, which would follow the English phrase, or TUC, which would follow the French phrase temps universel coordonné. The form UTC was adopted as a language-independent designation consistent with abbreviations used for related Universal Time scales, including UT0 and UT1.

UTC is frequently called “Greenwich Mean Time” in ordinary civil contexts because the two provide the same hour and minute for the Greenwich meridian at contemporary levels of precision. They are not identical technical concepts. GMT originated as an astronomical mean-solar scale, while UTC is an atomic scale maintained within a specified tolerance of UT1.

The designation “UTC time” is grammatically redundant but occurs in technical and administrative usage. An expression such as “14:00 UTC” identifies a reading on the UTC scale without implying a geographical time zone. The civil zone used in the United Kingdom during winter has the same numerical offset, but its legal basis is separate from the international definition of UTC.

Use in computing and communications

Computer systems commonly use UTC as the reference for timestamps exchanged across networks. The Network Time Protocol distributes estimates of UTC and includes mechanisms for indicating an announced leap second. Satellite navigation systems maintain their own continuous atomic scales and broadcast parameters relating those scales to UTC.

Unix time represents instants through a count associated with seconds since the beginning of 1 January 1970 UTC. Its treatment of leap seconds varies among specifications and implementations, so it is not a direct count of all SI seconds elapsed since the epoch. The ISO 8601 notation permits UTC to be identified by an offset of +00:00 or by the suffix Z, the latter being derived from the “zero” time-zone designator.

Legal and administrative time is established by individual jurisdictions. Many statutes define national time through an offset from UTC, while others retain terminology based on GMT or a national standard. These legal scales normally follow UTC in practice because national timing institutes connect their realizations to the international system.

Planned change to the adjustment system

The General Conference on Weights and Measures adopted a resolution in 2022 calling for the maximum permitted difference between UT1 and UTC to be increased by or before 2035. The change is intended to allow UTC to continue for an extended period without leap-second insertions. The numerical tolerance and the treatment of the accumulated difference remain matters of international metrological and radiocommunication coordination.

Under the revised arrangement, the UTC second continues to be the SI second, and UTC remains linked to TAI by a specified offset. The principal change concerns how closely civil time is required to track Earth rotation. Over sufficiently long intervals, a continuous atomic scale develops a measurable difference from mean solar time because the average terrestrial day exceeds 86,400 SI seconds.