Primary time standard
Coordinated Universal Time
(
UTC
) is the primary
time standard
globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for
civil time
and
time zones
. UTC facilitates international communication, navigation, scientific research, and commerce.
UTC has been widely embraced by most countries and is the effective successor to
Greenwich Mean Time
(GMT) in everyday usage and common applications.
[a]
In specialized domains such as scientific research, navigation, and timekeeping, other standards such as
UT1
and
International Atomic Time
(TAI) are also used alongside UTC.
UTC is based on TAI, which is a weighted average of hundreds of
atomic clocks
worldwide. UTC is within about one second of
mean solar time
at 0° longitude, the currently used
prime meridian
, and is not adjusted for
daylight saving time
.
The coordination of time and frequency transmissions around the world began on 1 January 1960. UTC was first officially adopted as a standard in 1963 and "UTC" became the official abbreviation of Coordinated Universal Time in 1967.
The current version of UTC is defined by the
International Telecommunication Union
.
Since adoption, UTC has been adjusted several times, notably adding
leap seconds
in 1972. Recent years have seen significant developments in the realm of UTC, particularly in discussions about eliminating leap seconds from the timekeeping system because leap seconds occasionally disrupt timekeeping systems worldwide. The General Conference on Weights and Measures adopted a resolution to alter UTC with a new system that would eliminate leap seconds by 2035.
[3]
Etymology
[
edit
]
The official abbreviation for Coordinated Universal Time is
UTC
. This abbreviation comes as a result of the
International Telecommunication Union
and the
International Astronomical Union
wanting to use the same abbreviation in all languages.
[4]
The compromise that emerged was
UTC
,
[5]
which conforms to the pattern for the abbreviations of the variants of Universal Time (UT0, UT1, UT2, UT1R, etc.).
McCarthy described the origin of the abbreviation:
In 1967 the
CCIR
adopted the names Coordinated Universal Time and Temps Universel Coordonne for the English and French names with the acronym UTC to be used in both languages. The name "Coordinated Universal Time (UTC)" was approved by a resolution of IAU Commissions 4 and 31 at the 13th General Assembly in 1967 (Trans. IAU, 1968).
Uses
[
edit
]
Time zones
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
, being fourteen hours ahead of UTC. In 1995, the island nation of
Kiribati
moved those of its atolls in the
Line Islands
from
UTC?10
to
UTC+14
so that Kiribati would all be on the same day.
UTC is used in many
Internet
and
World Wide 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
,
e.g. for
flight plans
and
air traffic control
. In this context it is frequently referred to as Zulu time, as described below.
Weather forecasts
and maps all use UTC to avoid confusion about time zones and daylight saving time. The
International Space Station
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
[
edit
]
UTC divides time into days, hours, minutes, and
seconds
. Days are conventionally identified using the
Gregorian calendar
, 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
, 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
.
[11]
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
(such as
UT1
) at
0° longitude
,
(at the
IERS Reference Meridian
). The
mean solar day
is slightly longer than 86,400 SI seconds so 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 fractional Julian days do not work properly with UTC.
Since 1972, UTC may be calculated by subtracting the accumulated leap seconds from
International Atomic Time
(TAI), which is a
coordinate time
scale tracking notional
proper time
on the rotating surface of the
Earth
(the
geoid
). In order to maintain a close approximation to
UT1
, 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 occurred only at the end of June or December. However, 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
or radio
time signals
. 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
(BIPM) monthly publication of tables of differences between canonical TAI/UTC and TAI(
k
)/UTC(
k
) as estimated in real-time by participating laboratories.
[18]
(See the article on
International Atomic Time
for details.)
Because of
time dilation
, a standard clock not on the geoid, or in rapid motion, will not maintain synchronicity with UTC. Therefore,
telemetry
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 impossible to compute the exact time interval elapsed between two UTC
timestamps
without consulting a table showing 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
[
edit
]
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 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 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 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
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
or
Reykjavik
in
Iceland
as they are always on UTC and do not currently use
daylight saving time
(which
Greenwich
and
London
do, and so could be a source of error).
[22]
Daylight saving time
[
edit
]
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.
History
[
edit
]
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 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.
The coordination of time and frequency transmissions around the world began on 1 January 1960. UTC was first officially adopted in 1963 as
CCIR
Recommendation 374,
Standard-Frequency and Time-Signal Emissions
, and "UTC" became the official abbreviation of Coordinated Universal Time in 1967.
In 1961, the
Bureau International de l'Heure
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.
to simplify future adjustments. This system was eventually approved as
leap seconds
in a new UTC in 1970 and implemented in 1972, along with the idea of maintaining the UTC second equal to the TAI second. 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."
As an intermediate step 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
1 January 1972 00:00:00 UTC
was
1 January 1972 00:00:10 TAI
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.
The current version of UTC is defined by
International Telecommunication 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 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
.
[41]
See the "
Current number of leap seconds
" section for the number of leap seconds inserted to date.
Current number of leap seconds
[
edit
]
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 July 2022
[update]
, there have been 27 leap seconds in total, all positive, putting UTC 37 seconds behind TAI.
A study published in March 2024 in
Nature
concluded that accelerated melting of ice in Greenland and Antarctica due to climate change has decreased Earth's rotational velocity, affecting UTC adjustments and causing problems for computer networks that rely on UTC.
[43]
Rationale
[
edit
]
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.
[46]
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 per 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 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
[
edit
]
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.
[49]
[50]
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.
[51]
In 2022 a resolution was 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.
[3]
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
seasons
relative to the yearly calendar that results from the calendar year not precisely matching the
tropical year
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,
[56]
but no permanent decision was reached; it only chose to engage in further study with the goal of reconsideration in 2023.
[57]
[
needs update
]
A proposed alternative to the leap second is the leap hour or leap minute, which requires changes only once every few centuries.
[58]
ITU World Radiocommunication Conference 2023 (WRC-23), which was held in Dubai (United Arab Emirates) from 20 November to 15 December 2023 formally recognized the
Resolution 4
of the 27th CGPM (2022) which decides that the maximum value for the difference (UT1-UTC) will be increased in, or before, 2035.
[59]
See also
[
edit
]
References
[
edit
]
Notes
[
edit
]
- ^
The pips are no longer broadcast from Greenwich, but from the National Physical Laboratory in Teddington, Surrey, which uses Coordinated Universal Time (UTC) - the successor of GMT - for its reading.
Citations
[
edit
]
General and cited sources
[
edit
]
- Allan, David W.; Ashby, Neil; Hodge, Clifford C. (1997).
The Science of Timekeeping
. Hewlett-Packard.
Application note
.
- Allen, Steve (2011a).
"UTC is doomed"
.
Archived
from the original on 4 December 2008
. Retrieved
18 July
2011
.
- Allen, Steve (2011b).
"UTC might be redefined without Leap Seconds"
.
Archived
from the original on 19 July 2011
. Retrieved
18 July
2011
.
- Arias, E. F.; Guinot, B.; Quinn, T. J. (29 May 2003).
Rotation of the Earth and Time scales
(PDF)
. ITU-R Special Rapporteur Group Colloquium on the UTC Time Scale.
- "Aviation Time"
.
AOPA's Path to Aviation
. Aircraft Owners and Pilots Association. 2006. Archived from
the original
on 27 November 2006
. Retrieved
17 July
2011
.
- "Bulletin C"
.
International Earth Rotation and Reference Systems Service
. 5 July 2022.
Archived
from the original on 27 April 2022
. Retrieved
18 June
2022
.
- Blair, Byron E., ed. (1974),
Time and Frequency: Theory and Fundamentals
(PDF)
, National Bureau of Standards, National Institute of Standards and Technology since 1988, p. 32,
archived
(PDF)
from the original on 1 March 2021
, retrieved
30 June
2019
- Chester, Geoff (15 June 2015).
"Wait a second... 2015 will be a little longer"
.
CHIPS: The Department of the Navy's Information Technology Magazine
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Archived
from the original on 12 February 2022
. Retrieved
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2021
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"Sandford Fleming and Universal Time"
.
Scientia Canadensis: Canadian Journal of the History of Science, Technology and Medicine
.
14
(1?2): 66?89.
doi
:
10.7202/800302ar
.
- Essen, L.
(1968).
"Time Scales"
(PDF)
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Metrologia
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4
(4): 161?165.
Bibcode
:
1968Metro...4..161E
.
doi
:
10.1088/0026-1394/4/4/003
.
S2CID
250771250
.
Archived
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18 October
2008
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- Evers, Liz (2013).
It's About Time: From Calendars and Clocks to Moon Cycles and Light Years - A History
. Michael O'Mara.
ISBN
978-1-78243-087-2
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- Finkleman, David; Allen, Steve; Seago, John; Seaman, Rob; Seidelmann, P. Kenneth (2011). "The Future of Time: UTC and the Leap Second".
American Scientist
.
99
(July?August 2011): 312.
arXiv
:
1106.3141
.
Bibcode
:
2011arXiv1106.3141F
.
doi
:
10.1511/2011.91.1
.
- Guinot, Bernard (August 2011). "Solar time, legal time, time in use".
Metrologia
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48
(4): S181?185.
Bibcode
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2011Metro..48S.181G
.
doi
:
10.1088/0026-1394/48/4/S08
.
S2CID
121852011
.
- "History of TAI-UTC"
. Time Service Dept.,
U.S. Naval Observatory
. c. 2009. Archived from
the original
on 19 October 2019
. Retrieved
4 January
2009
.
- Horzepa, Stan (17 September 2010).
"Surfin': Time for Ham Radio"
. American Radio Relay League.
Archived
from the original on 23 September 2010
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24 October
2011
.
- Howse, Derek (1997).
Greenwich Time and the Longitude
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ISBN
0-85667-468-0
.
- "How NTP Works"
.
NTP: The Network Time Protocol
. 28 July 2011.
Archived
from the original on 20 June 2014
. Retrieved
18 June
2022
.
See heading "NTP Timescale and Data Formats".
- "IAU resolutions adopted at the XVIth General Assembly, Grenoble, France, 1976"
(PDF)
. 1976.
Archived
(PDF)
from the original on 2 May 2019
. Retrieved
18 June
2022
.
Resolution no. 3 by Commissions 4 (Ephemerides/Ephemerides) 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".
- "Iceland"
. 2011. Archived from
the original
on 18 October 2011.
- International Earth Rotation and Reference Systems Service
(19 July 2011).
"IERS Bulletins"
.
Archived
from the original on 13 June 2022
. Retrieved
18 June
2022
.
- Irvine, Chris (18 December 2008).
"Scientists propose 'leap hour' to fix time system"
.
The Telegraph
. Archived from
the original
on 14 May 2011.
- ITU Radiocommunication Assembly (2002).
"Standard-frequency and time-signal emissions"
(PDF)
. International Telecommunication Union.
Archived
(PDF)
from the original on 27 April 2022
. Retrieved
2 August
2011
.
- Langley, Richard B. (20 January 1999).
"A Few Facts Concerning GMT, UT, and the RGO"
.
Archived
from the original on 16 July 2011
. Retrieved
17 July
2011
.
- "Leap second decision is postponed"
.
BBC News
. 19 January 2012.
Archived
from the original on 1 February 2019
. Retrieved
21 July
2018
.
- Markowitz, W.; Hall, R.; Essen, L.; Parry, J. (August 1958).
"Frequency of caesium in terms of Ephemeris Time"
(PDF)
.
Physical Review Letters
.
1
(3): 105?107.
Bibcode
:
1958PhRvL...1..105M
.
doi
:
10.1103/PhysRevLett.1.105
.
Archived
(PDF)
from the original on 19 October 2008
. Retrieved
18 October
2008
.
- Fleming, Sandford (1886).
"Time-reckoning for the twentieth century"
.
Annual Report of the Board of Regents of the Smithsonian Institution
(1): 345?366.
Archived
from the original on 5 October 2022
. Retrieved
23 July
2018
.
Reprinted in 1889:
Time-reckoning for the twentieth century
at the
Internet Archive
- Markowitz, Wm. (1988). "Comparisons of ET (Solar), ET (Lunar), UT and TDT". In Babcock, A. K.; Wilkins, G. A. (eds.).
The Earth's Rotation and Reference Frames for Geodesy and Geophysics: Proceedings of the 128th Symposium of the International Astronomical Union, held in Coolfont, West Virginia, U.S.A., 20?24 October 1986
. International Astronomical Union Symposia. Vol. 128. Dordrecht: Kluwer Academic Publishers. pp. 413?418.
Bibcode
:
1988IAUS..128..413M
.
ISBN
978-90-277-2657-5
.
- McCarthy, Dennis D.
(July 1991).
"Astronomical Time"
(PDF)
.
Proc. IEEE
.
79
(7): 915?920.
doi
:
10.1109/5.84967
.
Archived
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External links
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Look up
UTC
in Wiktionary, the free dictionary.