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 integer number of half-hours or quarter-hours from UT1 or UTC.
Current "civil time in a particular "time zone can be determined by adding or subtracting the number of hours and minutes specified by the "UTC offset, which ranges from "UTC−12:00 in the west to "UTC+14:00 in the east (see "List of UTC time offsets).
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 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 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 for letters used in addition to Z in qualifying time zones other than Greenwich.
On electronic devices that only allow the current time zone to be configured using maps or city names, UTC can be selected indirectly by selecting "Reykjavík, "Iceland, which is always on UTC and does not use daylight saving time.
Daylight saving time
UTC does not change with a change of seasons, but "local time 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.
At the 1884 "International Meridian Conference held in Washington, D.C., the local mean solar time at the "Royal Observatory, Greenwich in "England was chosen to define the Universal day, counted from 0 hours at mean midnight. This agreed with civil Greenwich Mean Time (GMT), used on the island of Great Britain since 1847. In contrast, astronomical GMT began at mean noon, 12 hours after mean midnight of the same date until 1 January 1925, whereas nautical GMT began at mean noon, 12 hours before mean midnight of the same date, at least until 1805 in the "Royal Navy, but persisted much later elsewhere because it was mentioned at the 1884 conference. In 1884, the Greenwich Meridian was used for two-thirds of all charts and maps as their "Prime Meridian. 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 signals were based on UT, and hence on the rotation of the Earth.
In 1955, the "caesium "atomic clock 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 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 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 the duration 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, cause the laws of motion to accurately predict the observed positions of solar system bodies. Within the limits of observing 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 work properly with the celestial laws of motion.
In 1961, the "Bureau International de l'Heure began coordinating the UTC process internationally (but the name Coordinated Universal Time was not adopted by the International Astronomical Union until 1967). Time steps occurred every few months thereafter, and frequency changes at the end of each year. The jumps increased in size to 100 ms. 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 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. M. R. Winkler both independently proposed that steps should be of 1 s 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, so that 1 January 1972 00:00:00 UTC was 1 January 1972 00:00:10 TAI exactly, making the difference between UTC and TAI an "integer number of seconds. 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. As of January 2017, there have been 27 leap seconds in total, all positive, putting UTC 37 seconds behind TAI.
Earth's "rotational speed is very slowly decreasing because of "tidal deceleration; this increases the length of the "mean solar day. 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. As a result, the SI second is close to 1/86400 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 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/cy, where cy means century). This rate fluctuates within the range of 1.7–2.3 ms/cy. While the rate due to "tidal friction alone is about 2.3 ms/cy, the "uplift of "Canada and "Scandinavia 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 1/800 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 2000s (decade), because of a slight acceleration of 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.
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.
Some time 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 be insufficient, and there will have to be leap seconds that are not at the end of a month. In a few tens of thousands of years (the timing is uncertain), LOD will exceed 86,401 s, causing UTC to require more than one leap second per day.
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.
There is a proposal to redefine UTC and abolish leap seconds, such that "sundials would slowly get further out of sync with civil time. 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. This would be a major practical change in civil timekeeping, but would take effect slowly 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 slowly eastward (reaching Paris and beyond). Thus, the time system would lose its fixed connection to the geographic coordinates based on the "IERS meridian. The difference between UTC and UT could reach 0.5 hour 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, convening on 2 November.
The possibility of suppressing the leap second was considered in November 2015 at the World Radiocommunication Conference (WRC-15), which is the international regulatory body which defines Coordinated Universal Time. No decision to suppress leap seconds was reached; the issue will be studied further and reconsidered in 2023.
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|Look up UTC in Wiktionary, the free dictionary.|
- Current UTC time
- Definition of Coordinated Universal Time in German law–ZeitG §1 (3)
- International Earth Rotation Service; list of differences between TAI and UTC from 1961 to present
- U.S. Naval Observatory: Systems of Time
- W3C Specification about UTC Date and Time and "IETF "Internet standard RFC 3339, based on "ISO 8601
- Standard of time definition: UTC, GPS, LORAN and TAI
- What is in a name? On the term Coordinated Universal Time