Coordinated Universal Time (UTC,--Fr. Temps Universel Coordonné) is International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the Earth's slowing rotation. French ( français,) is a Romance language spoken around the world by 118 million people as a native language and by about 180 to 260 million people International Atomic Time ( TAI, from the French name Temps Atomique International) is a high-precision atomic Time standard that tracks A leap second is a one- Second adjustment that keeps broadcast standards for time of day close to mean solar time. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Leap seconds are used to allow UTC to closely track UT1, which is mean solar time at the Royal Observatory, Greenwich. The Royal Observatory Greenwich (formerly the Royal Greenwich Observatory or RGO) was commissioned in 1675 by King Charles II, with the

The difference between UTC and UT1 cannot exceed 0. 9 s, so if high precision is not required the general term Universal Time (UT) (without a suffix) may be used.

In casual use, Greenwich Mean Time (GMT) is the same as UTC and UT1. Greenwich Mean Time ( GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London Owing to the ambiguity of whether UTC or UT1 is meant, and because timekeeping laws usually refer to UTC, GMT is avoided in careful writing.

Time zones around the world are expressed as positive or negative offsets from UTC. Local time is UTC plus the time zone offset for that location, plus an offset (typically +1) for daylight saving time, if in effect. Daylight saving time ( DST UTC replaced Greenwich Mean Time on 1 January 1972 as the basis for the main reference time scale or civil time in various regions. Greenwich Mean Time ( GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London New Year See also New Year The Ancient Romans began their consular year on January 1st since 153 BC Year 1972 ( MCMLXXII) was a Leap year starting on Saturday (link will display full calendar of the Gregorian calendar. In modern usage civil time refers to statutory time scales designated by civilian authorities or to local time indicated by clocks ^[1]

Example: Thursday, 2008-06-12 08:48 UTC This was the time in UTC when this page was last generated.

Abbreviation

Coordinated Universal Time is abbreviated UTC. The International Telecommunication Union wanted Coordinated Universal Time to have a single abbreviation for all languages. English speakers and French speakers each wanted the initials of their respective languages' terms to be used internationally: "CUT" for "coordinated universal time" and "TUC" for "temps universel coordonné". English is a West Germanic language originating in England and is the First language for most people in the United Kingdom, the United States French ( français,) is a Romance language spoken around the world by 118 million people as a native language and by about 180 to 260 million people This resulted in the final compromise of using "UTC". ^[2]

"UTC" also has the benefit that it fits in with the pattern for the abbreviations of variants of Universal Time. UT0, UT1, UT1R, and others exist, so appending "C" for "coordinated" to the base "UT" is very satisfactory for those who are familiar with the other types of UT.

"UTC" has been erroneously expanded into "Universal Time Code" or "Universal Time Convention". ^[3]

Mechanism

As a time scale, UTC divides up time into days, hours, minutes and seconds. A time standard Days are conventionally identified using the Gregorian calendar, but Julian day numbers can also be used. The Gregorian calendar is the most widely used Calendar in the world today The Julian date (JD is the interval of time in days and fractions of a day since 4713 B Each day contains 24 hours and each hour contains 60 minutes, but the number of seconds in a minute can be 60 or sometimes 61.

Most UTC days contain exactly 86,400 SI seconds, with exactly 60 seconds in each minute. The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units However, since the mean solar day is slightly longer than 86,400 SI seconds, occasionally the last minute of a UTC day will have 61 seconds. Solar times are measures of the apparent position of the Sun on the Celestial sphere. The extra second is called a leap second. A leap second is a one- Second adjustment that keeps broadcast standards for time of day close to mean solar time. 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 allowed to contain 59 seconds to cover the remote possibility of the Earth rotating faster, but that has never happened. The irregular day lengths mean that fractional Julian days do not work properly with UTC. The Julian date (JD is the interval of time in days and fractions of a day since 4713 B

UTC is derived from International Atomic Time (TAI), which is a time scale tracking proper time on the rotating surface of the Earth (the geoid). In relativity, proper time is Time measured by a single Clock between events that occur at the same place as the clock EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The geoid is that Equipotential surface which would coincide exactly with the mean ocean surface of the Earth if the oceans were in equilibrium at rest and extended through At any particular time, UTC proceeds as a linear function of TAI. In Mathematics, the term linear function can refer to either of two different but related concepts From 1972 onwards UTC ticks at the same rate as TAI, but earlier (back to the 1961 start of UTC) UTC ticked at a different rate from TAI. In order to remain a close approximation of UT1 (equivalent to GMT before 1960), UTC occasionally has discontinuities where it changes from one linear function of TAI to another. Greenwich Mean Time ( GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London These discontinuities take the form of leaps implemented by a UTC day of irregular length, and (prior to 1972) changes to the rate at which UTC ticks relative to TAI. Discontinuities in UTC have only ever occurred at the end of a Gregorian month. The Gregorian calendar is the most widely used Calendar in the world today ^[4]

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 range −0. "IERS" redirects here for other uses see IERS (disambiguation The International Earth Rotation and Reference Systems Service is The time correction DUT1 (sometimes also written DUT) is the difference between Universal Time ( UT1) which is defined by Earth's rotation 9 s < DUT1 < +0. 9 s. Since 1972 the discontinuities have consisted only of a leap of one second at the end of June 30 or December 31. Events 350 - Roman usurper Nepotianus, of the Constantinian dynasty, is defeated and killed by troops of the Usurper Events 406 – Vandals, Alans and Suebians cross the Rhine, beginning an invasion of Gallia. The IERS publishes its decision on whether to have a leap second on each of these dates a few months in advance, in Bulletin C. A leap second is a one- Second adjustment that keeps broadcast standards for time of day close to mean solar time. ^[5] In principle, leap seconds can also occur on March 31 or September 30, but the IERS has never found this necessary. Events 307 - After divorcing his wife Minervina, Constantine marries Fausta, the daughter of the retired Roman Emperor Events 1399 - Henry IV is proclaimed King of England. 1744 - France and Spain defeat the

As with TAI, UTC is only known with the highest precision in retrospect. The International Bureau of Weights and Measures (BIPM) publishes monthly tables of differences between canonical TAI/UTC and TAI/UTC as estimated in real time by participating laboratories. The International Bureau of Weights and Measures ( Bureau international des poids et mesures, in French) is an international Standards organization, one (See the article on International Atomic Time for details. International Atomic Time ( TAI, from the French name Temps Atomique International) is a high-precision atomic Time standard that tracks )

History

Originally, the local time at the Royal Observatory, Greenwich, England, was chosen as standard at the 1884 International Meridian Conference, leading to the widespread use of Greenwich Mean Time (GMT) in order to set local clocks. The Royal Observatory Greenwich (formerly the Royal Greenwich Observatory or RGO) was commissioned in 1675 by King Charles II, with the The International Meridian Conference was a conference held in October 1884 in Washington D Greenwich Mean Time ( GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London This location was chosen because by 1884 two-thirds of all charts and maps already used it as their Prime Meridian. The Prime Meridian is the meridian (line of Longitude) at which longitude is defined to be 0° In 1929, the term Universal Time (UT) was introduced to refer to GMT with the day starting at midnight. Until the 1950s, broadcast time signals were based on UT, and hence on the rotation of the Earth. A time signal is a visible audible mechanical or electronic signal used as a reference to determine the time of day

In 1955, the caesium atomic clock was invented. Caesium or cesium (ˈsiːziəm is the Chemical element with the symbol Cs and Atomic number 55 An atomic clock is a type of Clock that uses an Atomic resonance Frequency standard as its timekeeping element This provided a form of timekeeping that was both more stable and more convenient than astronomical observations. In 1956 the US National Bureau of Standards started to use atomic frequency standards in generating the WWV time signals. WWV is the Callsign of NIST 's Shortwave Radio station located in Fort Collins Colorado. 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. A millisecond (from Milli- and Second; abbreviation ms is one thousandth of a Second. Many such steps were used. The signal frequency was changed less often.

In 1958, the International Atomic Time (TAI) service started. It was based on the frequency for the caesium transition, newly established,^[6] that was later used to redefine the second in 1967. The WWV time signal's frequency was set to a simple offset from the TAI frequency: initially an offset of 1. 0 × 10^-8, so that WWV ticked exactly one second for every 1. 00000001 s of TAI.

Despite the initial controversy, it became clear that basing time signals on atomic clocks was an improvement over the prior system. However, it was widely desired to keep civil time synchronised with the Earth's rotation, and many uses of time signals (such as for navigation) relied on their closely matching Universal Time. In modern usage civil time refers to statutory time scales designated by civilian authorities or to local time indicated by clocks Navigation is the process of reading and controlling the movement of a craft or vehicle from one place to another WWV's compromise approach was copied by other agencies worldwide, such as the Royal Greenwich Observatory. It now became a concern that time signals should be synchronised with each other, rather than independently determining their own frequency offsets and phase shifts.

In 1960 an international agreement was made on atomic-based time signals. A Treaty is an agreement under International law entered into by actors in international law namely States and International organizations. A frequency offset of 1. 5 × 10^-8 was adopted by all the participating institutions, matching the then-current rate of UT2. Ad hoc phase shifts were used to synchronise the time signals as far as possible. It was determined that the Bureau International de l'Heure should henceforth choose the frequency offsets and coordinate the time steps. The Bureau International de l'Heure (BIH or the International Time Bureau, seated at the Paris Observatory, was the international bureau responsible for combining It was also decided to use larger jumps, of 50 ms instead of 20 ms.

UTC was officially initiated at the start of 1961. The TAI instant 1961-01-01T00:00:01. 422818 exactly was identified as UTC instant 1961-01-01T00:00:00. 000000 exactly, and UTC ticked exactly one second for every 1. 000000015 s of TAI. Time steps occurred every few months thereafter, and frequency changes at the end of each year. The jumps increased in size to 100 ms, with only one 50 ms jump having ever occurred. This UTC was intended to permit a very close approximation of UT2, within around 0. 1 s.

In 1967, the SI second was redefined in terms of the frequency supplied by a caesium atomic clock. This was the frequency that had been provisionally used in TAI since 1958. It was soon recognised 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 that it would be better for time signals to maintain a consistent frequency, and that that 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 200 ms 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. Louis Essen ( September 6, 1908 &ndash August 24, 1997) was an English Physicist whose most notable achievements were in M. R. Winkler both independently proposed that steps should be of 1 s only. ^[7] 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 0. 107758 TAI seconds exactly, so that 1972-01-01T00:00:00 UTC was 1972-01-01T00:00:10 TAI exactly, making the difference between UTC and TAI an integer number of seconds. The integers (from the Latin integer, literally "untouched" hence "whole" the word entire comes from the same origin but via French 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 which required a closer approximation of UT1 than UTC now provided.

The first leap second occurred on 1972-06-30. A leap second is a one- Second adjustment that keeps broadcast standards for time of day close to mean solar time. Year 1972 ( MCMLXXII) was a Leap year starting on Saturday (link will display full calendar of the Gregorian calendar. Events 350 - Roman usurper Nepotianus, of the Constantinian dynasty, is defeated and killed by troops of the Usurper Since then leap seconds have occurred on average once every 18 months, always on June 30 or December 31. Events 350 - Roman usurper Nepotianus, of the Constantinian dynasty, is defeated and killed by troops of the Usurper Events 406 – Vandals, Alans and Suebians cross the Rhine, beginning an invasion of Gallia. As of 2007 there have been 23 leap seconds in total, all positive, putting UTC 33 seconds behind TAI. It seems unlikely that a negative leap second will ever occur, but there is a small chance of one due to the acceleration of the Earth's crust in the 2000s. This acceleration has already led to the longest-ever period without a leap second, from 1999-01-01 to 2005-12-31. Year 1999 ( MCMXCIX) was a Common year starting on Friday (link will display full 1999 Gregorian calendar) New Year See also New Year The Ancient Romans began their consular year on January 1st since 153 BC Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. Events 406 – Vandals, Alans and Suebians cross the Rhine, beginning an invasion of Gallia.

Rationale

Graph showing the difference DUT1 between UT1 and UTC. The time correction DUT1 (sometimes also written DUT) is the difference between Universal Time ( UT1) which is defined by Earth's rotation Vertical segments correspond to leap seconds.

The Earth's rotational speed is very slowly decreasing due to tidal deceleration, causing the mean solar day to increase in length. Rotational speed (sometimes called speed of revolution) indicates for example how fast a motor is running Tidal acceleration is an effect of the Tidal forces between an orbiting Natural satellite ( i Solar times are measures of the apparent position of the Sun on the Celestial sphere. The length of the SI second was based on the mean solar day observed between 1750 and 1892, analysed by Simon Newcomb. Simon Newcomb ( March 12 1835 &ndash July 11 1909) was a Canadian American Astronomer and Mathematician As a result, the SI second was exactly 1/86400 of a mean solar day in around 1820. In earlier centuries the mean solar day was shorter than 86400 SI seconds, and in later centuries it is longer than 86400 seconds. At 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. 002 s. For this reason, UT is now "slower" than TAI.

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. At the end of the 20th century, with the LOD at 2 ms above the nominal value, UTC ran faster than UT by 2 ms per day, getting a second ahead roughly every 500 days. Thus leap seconds were inserted at approximately this interval, retarding UTC to keep it synchronised in the long term. Note that the actual rotational period varies on unpredictable factors such as tectonic motion and has to be observed rather than computed. The rotation period of an astronomical object is the time it takes to complete one revolution around its Axis of rotation relative to the background stars Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere

The insertion of a leap second every 500 days does not mean that the mean solar day is getting longer by a second every 500 days: it will take approximately 50,000 years for a mean solar day to lengthen by one second (at a rate of 2 ms/cy). This is a mean rate within the range of 1. 7–2. 3 ms/cy. The rate due to tidal friction alone is about 2. Tidal acceleration is an effect of the Tidal forces between an orbiting Natural satellite ( i 3 ms/cy, but the uplift of Canada and Scandinavia by several metres since the last ice age has temporarily reduced this to 1. Country to "Dominion of Canada" or "Canadian Federation" or anything else please read the Talk Page Terminology and usage As a cultural term "Scandinavia" has no official definition and is subject to usage by those who identify with the culture in question as well An ice age is a period of long-term reduction in the Temperature of the Earth 's surface and atmosphere resulting in an expansion of continental Ice sheets 7 ms/cy over the last 2700 years. ^[8] The correct reason for leap seconds is not the current difference between actual and nominal LOD, but rather the accumulation of this difference over a period of time: in the late twentieth century, this difference was about 1/500 of a second, so it accumulated to 1 second after about 500 days.

For example, assume you start counting the seconds from the Unix epoch of 1970-01-01T00:00:00 UTC with an atomic clock. Unix time, or POSIX time, is a system for describing points in Time, defined as the number of seconds elapsed since Midnight Coordinated Universal At midnight on that day (as measured on UTC), your counter registers 0 s. After Earth has made one full rotation with respect to the mean Sun, your counter will register approximately 86400. 002 s (the precise value will vary depending on plate tectonic conditions). Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere Based on your counter, you can calculate that the date is 1970-01-02T00:00:00 UT1. After 500 rotations, your counter will register 43,200,001 s. Since 86,400 s × 500 is 43,200,000 s, you will calculate that the date is 1971-05-16T00:00:01 UTC, while it is only 1971-05-16T00:00:00 UT1. If you had added a leap second on December 31, 1970, retarding your counter by 1 s, then the counter would have a value of 43,200,000 s at 1971-05-16T00:00:00 UT1, and allow you to calculate the correct date. Events 406 – Vandals, Alans and Suebians cross the Rhine, beginning an invasion of Gallia. Year 1970 ( MCMLXX) was a Common year starting on Thursday (link shows full calendar of the Gregorian calendar.

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 time correction DUT1 (sometimes also written DUT) is the difference between Universal Time ( UT1) which is defined by Earth's rotation (Note that the slope became shallower in the 2000s, due to a slight acceleration of the Earth's crust 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 this graph. The frequency of leap seconds therefore corresponds to the slope of the diagonal graph segments, and thus to the excess LOD.

Future

See also: leap second

As the Earth's rotation continues to slow, positive leap seconds will be required more frequently. A leap second is a one- Second adjustment that keeps broadcast standards for time of day close to mean solar time. The long-term rate of change of LOD is approximately +1. In Calculus, a branch of mathematics the derivative is a measurement of how a function changes when the values of its inputs change 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.

Sometime in the 22nd century, two leap seconds will be required every year. The current use of only the leap second opportunities in June and December will be insufficient, and the March and September options will have to be used. In the 25th century, four leap seconds will be required every year, so the current quarterly options will be insufficient. Thereafter there will need to be the possibility of leap seconds at the end of any month. In about two thousand years even that will become insufficient, and there will have to be leap seconds that are not at the end of a month. ^[9]

In a few tens of thousands of years (the timing is very uncertain) LOD will exceed 86,401 s, causing the current form of UTC to break down due to requiring more than one leap second per day. It would be possible to then continue with double leaps, but this becomes increasingly untenable.

Both the one-leap-second-per-month and one-leap-second-per-day milestones are considered (by different theorists) to mark the theoretical limit of the applicability of UTC. The actual number of leap seconds to keep track of time would become unwieldy by current standards well before these, but presumably if UTC were to continue then horological systems would be redesigned to cope with regular leap seconds much better than current systems do.

There is a proposal to redefine UTC and abolish leap seconds, such that sundials would slowly get further out of sync with civil time. A sundial is a device that measures time by the position of the Sun. In modern usage civil time refers to statutory time scales designated by civilian authorities or to local time indicated by clocks ^[10] The resulting gradual shift of the sun's movements relative to civil time is analogous to the shift of seasons relative to the yearly calendar that results from the calendar year not precisely matching the tropical year length. A season is one of the major divisions of the Year, generally based on yearly periodic changes in Weather. A tropical year (also known as a solar year) is the length of time that the Sun takes to return to the same position in the cycle of seasons as seen from Earth This would be a major practical change in civil timekeeping, but would take effect slowly over several centuries. An ITU study group is to vote on this possibility during 2008, possibly leading to official approval by the World Radio Conference in 2011 and the cessation of leap seconds in 2013.

There is also a proposal that the present form of UTC could be improved to track UT1 more closely, by allowing greater freedom in scheduling leap seconds. ^[11]

Uses

UTC is the time system used for many Internet and World Wide Web standards. The Internet is a global system of interconnected Computer networks The World Wide Web (commonly shortened to the Web) is a system of interlinked Hypertext documents accessed via the Internet. In particular, the Network Time Protocol, which is designed to synchronize the clocks of many computers over the Internet (usually to that of a known accurate atomic clock), uses UTC. The Network Time Protocol ( NTP) is a protocol for distributing the Coordinated Universal Time (UTC by means of synchronizing the clocks of computer systems

Those who transmit on the amateur radio bands often log the time of their radio contacts in UTC, as transmissions can go worldwide on some frequencies. Amateur radio, often called ham radio, is both a Hobby and a service in which participants called "hams" use various types of Radio communications In the past, the FCC required all amateur radio operators in the United States to log their radio conversations. See also Amateur radio An amateur radio operator is an individual who typically uses equipment at an Amateur radio station to engage in two-way The United States of America —commonly referred to as the A Logbook was originally a book for recording readings from the log, and is used to determine the distance a ship traveled within a certain amount of time

UTC is also the time system used in aviation, referred to as Zulu. Aviation refers to activities involving man-made flying devices ( Aircraft) including the people organizations and regulatory bodies involved with them ^[12] Weather reports, flight plans, air traffic control clearances, and maps all use UTC to avoid confusion about time zones and daylight saving time. Weather Report was an influential Jazz fusion band of the 1970s and early 1980s combining Jazz and Latin jazz with Art music, Flight plans are documents filed by pilots or a Flight Dispatcher with the local Civil Aviation Authority (e Air traffic control ( ATC) is a service provided by ground-based controllers who direct Aircraft on the ground and in the air Daylight saving time ( DST

Because of time dilation, a standard clock not on the geoid, or in rapid motion, will not maintain synchronicity with UTC. This article discusses a concept in physics For the concept in sociology see Time displacement. The geoid is that Equipotential surface which would coincide exactly with the mean ocean surface of the Earth if the oceans were in equilibrium at rest and extended through Therefore, telemetry from clocks with a known relation to the geoid is used to provide UTC, when required, on locations such as that of spacecraft. Telemetry (synonymous with Telematics) is a Technology that allows the remote measurement and reporting of Information of interest to the system designer

UTC is a discontinuous timescale, so it is not possible to compute the exact time interval elapsed between two UTC timestamps without consulting a table that describes how many leap seconds occurred during that interval. For other uses see Time (disambiguation Time is a component of a measuring system used to sequence events to compare the durations of Therefore, many scientific applications that require precise measurement of long (multi-year) intervals use TAI instead. International Atomic Time ( TAI, from the French name Temps Atomique International) is a high-precision atomic Time standard that tracks TAI is also commonly used by systems that can not handle leap seconds. A fixed 19-second offset from TAI also gives GPS time. Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth

For most common and legal-trade purposes, the fractional second difference between UTC and UT (GMT) is inconsequentially small, so UTC is often called GMT, for example by the BBC, although that usage is not technically correct. Greenwich Mean Time ( GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London ^[13]

Time zones

Main articles: Time zone and List of time zones

Time zones usually differ from UTC by an integral number of hours^[14], although the laws of each jurisdiction would have to be consulted if sub-second accuracy was required. This is a list of Time zones sorted by Time offsets from Coordinated Universal Time (UTC Several jurisdictions established time zones that differ by an integer number of half-hours from UT1 or UTC. ^[15]

The UTC time zone is sometimes denoted by the letter Z – a reference to the equivalent nautical time zone (GMT), which has been denoted by a Z since about 1950. Greenwich Mean Time ( GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London 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 and amateur radio word for Z is "Zulu", UTC is sometimes known as Zulu time. The NATO phonetic alphabet, more formally the international radiotelephony spelling alphabet, is the most widely used Spelling alphabet. Amateur radio, often called ham radio, is both a Hobby and a service in which participants called "hams" use various types of Radio communications This is especially true in aviation, where Zulu is the universal standard. ^[16] This ensures all pilots regardless of location are using the same 24-hour clock, thus avoiding confusion when flying between time zones. Description A time of day is written in the 24-hour notation in the form hhmm (for example 0123 or hhmmss (for example 012345 where hh (00 to 23 is the decimal number ^[17][18]

See also

• Ephemeris Time
• Terrestrial Time
• ISO 8601

Bibliography

• Allan, David W. Ephemeris Time ( ET) is a time scale used in Ephemerides of celestial bodies in particular the Sun (as observed from the Earth Moon planets and other members of Terrestrial Time (TT is the modern astronomical standard for the passage of time on the surface of the Earth (for civil purposes Coordinated Universal ISO 8601 is an International standard for date and Time representations issued by the International Organization for Standardization (ISO , Neil Ashby, Clifford C. Hodge: The Science of Timekeeping. Hewlett Packard Application Note 1289, 1997. A datasheet is a Document summarizing the performance and other characteristics of a component (e
• ITU-R Recommendation TF. The ITU Radiocommunication Sector ( ITU-R) is one of the three sectors (divisions or units of the International Telecommunication Union (ITU and is responsible for 460-4: Standard-frequency and time-signal emissions. International Telecommunication Union. (Annex I of this document contains the official definition of UTC. )
• McCarthy, Dennis D. : "Astronomical Time". Proc. IEEE, Vol. 79, No. 7, July 1991, pp. Year 1991 ( MCMXCI) was a Common year starting on Tuesday (link will display full calendar of the Gregorian Calendar. 915-920.
• Nelson, McCarthy, et al. : "The leap second: its history and possible future"PDF (381 KiB), Metrologia, Vol. A kibibyte (a contraction of ki lo bi nary byte) is a unit of Information or Computer storage, established by the International 38, pp. 509–529, 2001.
• Seidelman, P. K. (ed). Explanatory Supplement to the Astronomical Almanac. Sausalito, CA: University Science Books. 1992.

References

External links

• Bureau International des Poids et Mesures UTC/TAI Time Server
• The official U.S. time in UTC zone using Java. Java refers to a number of Computer software products and specifications from Sun Microsystems that together provide a system for developing Application software
• World Time Server – any location, any time
• thetimeNOW – Current time in all time zones
• United States Naval Observatory – What is Universal Time?
• International Earth Rotation Service Leap Second Updates
• W3C Specification about UTC Date and Time and IETF Internet standard RFC 3339, based on ISO 8601
• Zulu Time
• Hong Kong Time by Hong Kong Observatory
• Alpha to Zulu time zones
• Definition of Coordinated Universal Time in German law – ZeitG §1 (3)
• The Science of Timekeeping -- by David W. In Computer network Engineering, an Internet Standard (STD is a Specification, put forward by the Internet Engineering Task Force (IETF for ISO 8601 is an International standard for date and Time representations issued by the International Organization for Standardization (ISO Allan of Allan variance fame