The time correction DUT1 (sometimes also written DUT) is the difference between Universal Time (UT1), which is defined by Earth's rotation, and Coordinated Universal Time (UTC), which is defined by a network of precision atomic clocks.

DUT1 = UT1 − UTC

UTC is maintained via leap seconds, such that DUT1 remains within the range −0.9 s < DUT1 < +0.9 s. The reason for this correction is partly that the rate of rotation of the Earth is not constant, due to tidal braking and the redistribution of mass within the Earth, including its oceans and atmosphere, and partly because the SI second (as now used for UTC) was already, when adopted, a little shorter than the current value of the second of mean solar time.[1]

Forecast values of DUT1 are published by IERS Bulletin A.

Weekly updated values of DUT1 with 0.1 s precision are broadcast by several time signal services, including WWV and MSF. These services transmit one pulse per second of some sort. To represent positive DUT1 values from +0.1 to +0.8 seconds, the pulses sent during seconds 1 through 8 are "emphasized" in some way, generally by transmitting a double pulse. The number of emphasized pulses gives the value of DUT1. Negative DUT1 values, from −0.1 to −0.8 seconds, are similarly represented by emphasizing pulses 9 through 16. For example, a DUT1 value of −0.4 would be transmitted by emphasizing pulses 9 through 12.

The Russian time signal RWM transmits an additional correction dUT1 in 0.02 s increments. Positive values of dUT1 from +0.02 to +0.08 s are encoded by emphasizing pulses 21 through 24; negative values are encoded by emphasizing pulses 31 through 34. The actual value of DUT1 is approximated by the sum of the transmitted DUT1 + dUT1.

The longwave RBU time signal also transmits dUT1.

Graph showing the difference in seconds between UT1 and UTC over time. A vertical transition indicates the use of a leap second. The red portion of the graph indicates predicted values.

References and notes

  1. ^ :(1) In "The Physical Basis of the Leap Second", by D D McCarthy, C Hackman and R A Nelson, in Astronomical Journal, vol.136 (2008), pages 1906–1908, it is stated (page 1908), that "the SI second is equivalent to an older measure of the second of UT1, which was too small to start with and further, as the duration of the UT1 second increases, the discrepancy widens." :(2) In the late 1950s, the cesium standard was used to measure both the current mean length of the second of mean solar time (UT2) (result: 9192631830 cycles) and also the second of ephemeris time (ET) (result: 9192631770±20 cycles), see "Time Scales", by L. Essen, in Metrologia, vol.4 (1968), pp.161–165, on p.162. As is well known, the 9192631770 figure was chosen for the SI second. L Essen in the same 1968 article (p.162) stated that this "seemed reasonable in view of the variations in UT2".

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Apparent place

The apparent place of an object is its position in space as seen by an observer. Because of physical and geometrical effects it may differ from the "true" or "geometric" position.

BSF (time service)

BSF is the callsign of the time signal transmitter for Taiwan, which transmits time information on 77.5 kHz in the longwave range, and 5 MHz & 15 MHz in the shortwave range from Chung-Li. The longwave transmitter, which uses a T-antenna is situated at 24°03′59″N 120°25′18″E.

Due to "low demand", the short wave of BSF was discontinued as of July 1, 2004. The time signal is currently transmitted at a low frequency of 77.5 kHz

CHU (radio station)

CHU is the call sign of a shortwave time signal radio station operated by the Institute for National Measurement Standards of the National Research Council of Canada.

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.


DUT is an acronym for

Da Nang University of Technology in Da Nang, Vietnam

Dalian University of Technology in Dalian, China

Delft University of Technology in Delft, the Netherlands

Device under test, especially in electronics, associated with semiconductor testing

Diplôme universitaire de technologie, a French undergraduate university degree in technology

Durban University of Technology in Durban, South Africa

DUT (gene), which encodes DUTP pyrophosphatase

Drinking-up time, a former feature of United Kingdom alcohol licensing law

DUT1 or DUT, which describes the difference between coordinated universal time (UTC) and universal time (UT)

Unalaska Airport (IATA airport code "DUT"), an airport in Unalaska, Alaska, U.S.

D.Ut., an abbreviation used for the United States District Court for the District of Utah

Double Irish arrangement

The Double Irish is a base erosion and profit shifting ("BEPS") corporate tax tool, used mostly by US multinationals since the late 1980s, to avoid corporate taxation on most non–U.S. profits. It is the largest tax avoidance tool in history and by 2010, was shielding US$100 billion annually in US multinational foreign profits from taxation, and was the main tool by which US multinationals built up untaxed offshore reserves of US$1 trillion from 2004 to 2018. Traditionally, it was also used with the Dutch Sandwich BEPS tool; however, changes to Irish tax law in 2010 dispensed with this requirement for most users.

Despite US knowledge about the Double Irish for a decade, it was the EU who forced Ireland to close the scheme in October 2014, starting January 2015. However, users of existing schemes, such as Apple, Google, Facebook, and Pfizer, were given until January 2020 to close them. At the announcement of the closure it was known that Ireland had replacement BEPS tools, the Single Malt (2014), and the Capital Allowances for Intangible Assets (CAIA) (2009):

US tax academics showed as far back as 1994, that US multinational use of tax havens and BEPS tools had maximised long-term US exchequer receipts. They showed that multinationals from "territorial" tax systems, which all but a handful of countries follow, did not use BEPS tools, or tax havens, including those that had recently switched, such as Japan (2009), and the UK (2009–12). By 2018, non–US tax academics showed US multinationals were the largest users of BEPS tools and Ireland was the largest global BEPS hub or tax haven. They showed Ireland was almost exclusively a US corporate tax haven, and that US multinationals represented the largest component of the Irish economy, and that Ireland had failed to attract multinationals from "territorial" tax systems.US tax academics advocated the US switch to a "territorial" tax system in the December 2017 Tax Cuts and Jobs Act ("TCJA"), and as a result, forecast the demise of Irish BEPS tools, and Ireland as a US corporate tax haven. However, by mid–2018, other tax academics, including the IMF, noted technical flaws in the TCJA had increased the attractiveness of Ireland's BEPS tools, and the CAIA BEPS tool in particular, which post-TCJA, delivered a total effective tax rate ("ETR") of 0–2.5% on profits that can be fully repatriated to the US without incurring any additional US taxation. In July 2018, one of Ireland's leading tax economists forecasted a "boom" in the use of the Irish CAIA BEPS tool, as US multinationals close existing Double Irish BEPS schemes before the 2020 deadline.

HLA (radio station)

HLA is a time signal radio station in Daejeon, South Korea, operated by the Korea Research Institute of Standards and Science. Established on November 24, 1984, it transmits a 2 kW signal on 5 MHz (±0.01 Hz). Originally only transmitted for 7 hours per day (01:00–08:00), 5 days per week (M–F), it is continuous as of 2011. There are over 100 users of the signal in Korea.It broadcasts a time signal similar to that of the WWV and WWVH stations with which it shares a frequency:

Second pulses are 5 ms (9 cycles) of 1800 Hz, beginning on the second

Seconds 29 and 59 are omitted

Minute markers are 800 ms of the same frequency

Hour markers are 800 ms of 1500 Hz

DUT1 is encoded using doubled pulses

Voice time announcements are made after second 52

A time code is transmitted on a 100 Hz subcarrier


An hourglass (or sandglass, sand timer, sand clock or egg timer) is a device used to measure the passage of time. It comprises two glass bulbs connected vertically by a narrow neck that allows a regulated trickle of material (historically sand) from the upper bulb to the lower one. Factors affecting the time it measured include sand quantity, sand coarseness, bulb size, and neck width. Hourglasses may be reused indefinitely by inverting the bulbs once the upper bulb is empty. Depictions of hourglasses in art survive in large numbers from antiquity to the present day, as a symbol for the passage of time. These were especially common sculpted as epitaphs on tombstones or other monuments, also in the form of the winged hourglass, a literal depiction of the well-known Latin epitaph tempus fugit ("time flies").

International Earth Rotation and Reference Systems Service

The International Earth Rotation and Reference Systems Service (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter (EOP) and International Celestial Reference System (ICRS) groups.

Leap second

A leap second is a one-second adjustment that is occasionally applied to civil time Coordinated Universal Time (UTC) to keep it close to the mean solar time at Greenwich, in spite of the Earth's rotation slowdown and irregularities. UTC was introduced on January 1, 1972, initially with a 10 second lag behind International Atomic Time (TAI). Since that date, 27 leap seconds have been inserted, the most recent on December 31, 2016 at 23:59:60 UTC, so in 2018, UTC lags behind TAI by an offset of 37 seconds.The UTC time standard, which is widely used for international timekeeping and as the reference for civil time in most countries, uses the international system (SI) definition of the second. The UTC second has been calibrated with atomic clocks to the duration of the Earth's mean day of the astronomical year 1900. Because the rotation of the Earth has since further slowed down, the duration of today's mean solar day is longer (by roughly 0.001 seconds) than 24 SI hours (86,400 SI seconds). UTC would step ahead of solar time and need adjustment even if the Earth's rotation remained constant in the future. Therefore, if the UTC day were defined as precisely 86,400 SI seconds, the UTC time-of-day would slowly drift apart from that of solar-based standards, such as Greenwich Mean Time (GMT) and its successor UT1. The point on the Earth's equator where the sun culminates at 12:00:00 UTC would wander to the East by some 300 m each year. The leap second compensates for this drift, by occasionally scheduling a UTC day with 86,401 or (in principle) 86,399 SI seconds.

When it occurs, a positive leap second is inserted between second 23:59:59 of a chosen UTC calendar date and second 00:00:00 of the following date. The definition of UTC states that the last day of December and June are preferred, with the last day of March or September as second preference, and the last day of any other month as third preference. All leap seconds (as of 2017) have been scheduled for either June 30 or December 31. The extra second is displayed on UTC clocks as 23:59:60. On clocks that display local time tied to UTC, the leap second may be inserted at the end of some other hour (or half-hour or quarter-hour), depending on the local time zone. A negative leap second would suppress second 23:59:59 of the last day of a chosen month, so that second 23:59:58 of that date would be followed immediately by second 00:00:00 of the following date. Since the introduction of leap seconds, the mean solar day has outpaced UTC only for very brief periods, and has not triggered a negative leap second.

Because the Earth's rotation speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and unpredictable. Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS), when needed to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 seconds.

RBU (radio station)

RBU is a time code radio station located in Moscow (56°44′00″N 37°39′48″E). It transmits a continuous 10 kW time code on 66⅔ kHz. This is commonly written as 66.66 or 66.666 kHz, but is actually 200/3 = 66.6̅ kHz. Until 2008, the transmitter site was near Kupavna 55°44′04″N 38°9′0″E and used as antenna three T-antennas spun between three 150 metres tall grounded masts. In 2008, it has been transferred to the Taldom transmitter at 56°44′00″N 37°39′48″E.RBU is controlled by All-Russian Scientific Research Institute for Physical-Engineering and Radiotechnical Metrology. It is operated by Russian Television and Radio Broadcasting Network.


RWM is the callsign of a high frequency (shortwave) standard frequency and time signal radio station in Moscow, Russia. It is controlled by All-Russian Scientific Research Institute for Physical-Engineering and Radiotechnical Metrology, and operated by Russian Television and Radio Broadcasting Network. Transmitting frequencies are 4.996 MHz with 5 kW and on 9.996 and 14.996 MHz with 8 kW.

The frequencies are very close to those of WWV, and WWVH. Because of this, RWM is very difficult to receive in North America with simple receivers of low selectivity, due to interference from these said stations.

The mode of transmission is N0N and A1A (CW). Between 0 and 8 minutes past the hour, RWM transmits a straight unmodulated carrier wave. At 9 minutes past, RWM identifies itself in Morse code. Between 10 and 20 minutes past the hour, RWM transmits a pulse of carrier every second, with the difference between UT1 and UTC in units of one-fiftieth of a second encoded onto the once-per-second pulses. Between 20 and 30 minutes past the hour, RWM transmits 10 carrier pulses each second. This transmission cycle is repeated every half-hour.RWM does not transmit the time of day, only standard time intervals.

The 1 Hz pulses begin on the second, and are doubled (a second pulse transmitted from 200–300 ms past the second) to encode DUT1 and dUT1. Using these values, UT1 may be computed as:

UT1 = UTC + DUT1 × 0.1 s + dUT1 × 0.02 sDUT1 may vary between −8 and +8. The number of double pulses sent during seconds 1–8 of each minute encode positive values; if DUT1 = +5, then pulses 1 through 5 will be doubled. Doubling pulses 9–16 encodes negative values similarly.

dUT1 varies from −4 to +4. Positive values are encoded by double pulses during seconds 21–24 of each minute. Negative values are encoded during seconds 31–34.The 10 Hz pulses are widened in a pattern similar to that of the Beta time signal: Most pulses are 20 ms, but ones sent on the second are 40 ms, and ones sent on the minute are 500 ms.

Term (time)

A term is a period of duration, time or occurrence, in relation to an event. To differentiate an interval or duration, common phrases are used to distinguish the observance of length are near-term or short-term, medium-term or mid-term and long-term.

It is also used as part of a calendar year, especially one of the three parts of an academic term and working year in the United Kingdom: Michaelmas term, Hilary term / Lent term or Trinity term / Easter term, the equivalent to the American semester. In America there is a midterm election held in the middle of the four-year presidential term, there are also academic midterm exams.

In economics, it is the period required for economic agents to reallocate resources, and generally reestablish equilibrium. The actual length of this period, usually numbered in years or decades, varies widely depending on circumstantial context. During the long term, all factors are variable.

In finance or financial operations of borrowing and investing, what is considered long-term is usually above 3 years, with medium-term usually between 1 and 3 years and short-term usually under 1 year. It is also used in some countries to indicate a fixed term investment such as a term deposit.

In law, the term of a contract is the duration for which it is to remain in effect (not to be confused with the meaning of "term" that denotes any provision of a contract). A fixed-term contract is one concluded for a pre-defined time, although it may also include provision for it to be extended. A contractor required to deliver against a term contract is often referred to as a "term contractor".


Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past, through the present, to the future. Time is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience. Time is often referred to as a fourth dimension, along with three spatial dimensions.Time has long been an important subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars.

Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective measuring systems.Time in physics is unambiguously operationally defined as "what a clock reads". See Units of Time. Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities. Time is used to define other quantities – such as velocity – so defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. The operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy.

Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms (see below). Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human life spans.

Time from NPL (MSF)

The Time from NPL is a radio signal broadcast from the Anthorn Radio Station near Anthorn, Cumbria, which serves as the United Kingdom's national time reference. The time signal is derived from three atomic clocks installed at the transmitter site, and is based on time standards maintained by the UK's National Physical Laboratory (NPL) in Teddington. The service is provided by Babcock International (which acquired former providers VT Communications), under contract to the NPL. It was funded by the former Department for Business, Innovation and Skills; as of 2017 NPL Management Limited (NPLML) was owned by the Department for Business, Energy and Industrial Strategy (BEIS), and NPL operated as a public corporation.The signal, also known as the MSF signal (and formerly the Rugby clock), is broadcast at a highly accurate frequency of 60 kHz and can be received throughout the UK, and in much of northern and western Europe.

The signal's carrier frequency is maintained at 60 kHz to within 2 parts in 1012, controlled by caesium atomic clocks at the radio station.

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). 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.


WWVB is a time signal radio station near Fort Collins, Colorado and is operated by the National Institute of Standards and Technology (NIST). Most radio-controlled clocks in North America use WWVB's transmissions to set the correct time. The 70 kW ERP signal transmitted from WWVB is a continuous 60 kHz carrier wave, the frequency of which is derived from a set of atomic clocks located at the transmitter site, yielding a frequency uncertainty of less than 1 part in 1012. A one-bit-per-second time code, which is based on the IRIG "H" time code format and derived from the same set of atomic clocks, is then modulated onto the carrier wave using pulse-width modulation and amplitude-shift keying. A single complete frame of time code begins at the start of each minute, lasts one minute, and conveys the year, day of year, hour, minute, and other information such as the beginning of the minute.

WWVB is co-located with WWV, a time signal station that broadcasts in both voice and time code on multiple shortwave radio frequencies.

While most time signals encode the local time of the broadcasting nation, the United States spans multiple time zones, so WWVB broadcasts the time in Coordinated Universal Time (UTC). Radio-controlled clocks can then apply time zone and daylight saving time offsets as needed to display local time. The time used in the broadcast is set by the NIST Time Scale, known as UTC(NIST). This time scale is the calculated average time of an ensemble of master clocks, themselves calibrated by the NIST-F1 and NIST-F2 cesium fountain atomic clocks.In 2011, NIST estimated the number of radio clocks and wristwatches equipped with a WWVB receiver at over 50 million.WWVB, along with NIST's shortwave time code-and-announcement stations WWV and WWVH, were proposed for defunding and elimination in the 2019 NIST budget. However, the final 2019 NIST budget preserved funding for the three stations.

WWV (radio station)

WWV is a shortwave (also known as "high frequency" (HF)) radio station, located near Fort Collins, Colorado. It is best known for its continuous time signal broadcasts begun in 1945, and is also used to establish official United States government frequency standards, with transmitters operating on 2.5, 5, 10, 15, and 20 MHz. WWV is operated by U.S. National Institute of Standards and Technology (NIST), under the oversight of its Time and Frequency Division, which is part of NIST's Physical Measurement Laboratory based in Gaithersburg, Maryland.WWV was first established in 1919 by the Bureau of Standards in Washington, D.C., and has been described as the oldest continuously-operating radio station in the United States. In 1931 it relocated to the first of three suburban Maryland sites, before moving to its current location near Fort Collins in 1966. WWV shares this site with longwave (also known as "low frequency" (LF)) station WWVB, which transmits carrier and time code (no voice) at 60 kHz.

NIST also operates the similarly structured WWVH on Kauai, Hawaii. Both WWV and WWVH announce the Coordinated Universal Time each minute, and make other recorded announcements of general interest on an hourly schedule, including the Global Positioning System (GPS) satellite constellation status. Because they simultaneously transmit on the same frequencies, WWV uses a male voice in order to differentiate itself from WWVH, which uses a female voice.

NIST has announced plans to celebrate WWV's centennial on October 1, 2019. (See web site for celebration plans:http://wwv100.com/) WWV, along with WWVB and WWVH, was recommended for defunding and elimination in NIST's Fiscal Year 2019 budget request. However, the final 2019 NIST budget preserved funding for the three stations.

International standards
Obsolete standards
Time in physics
Archaeology and geology
Astronomical chronology
Other units of time
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