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{{short description|Extra second inserted to keep civil time in sync with the Earth's rotation}}{{Use mdy dates|date=June 2017}}File:Leapsecond2016.png|thumb|right|300px|Screenshot of the UTC clock from {{URLweblink}} during the leap second on December 31, 2016. In the U.S., the leap second took place at 18:59:60 local time on the East Coast, at 15:59:60 local time on the West Coast, and at 13:59:60 local time in Hawaii.]]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.WEB,weblink Bulletin C 52, International Earth Rotation and Reference Systems Service, IERS, Bizouard, Christian, July 6, 2016, no,weblink December 30, 2016, mdy-all, 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 clock on 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 ({{formatnum:86400}} 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 {{formatnum:86400}} 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 {{formatnum:86401}} or (in principle) {{formatnum:86399}} 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.WEB,weblink International Telecommunications Union Radiocommunications sector recommendation TF.460-6: Standard-frequency and time-signal emissions, February 9, 2017, no,weblink October 17, 2016, mdy-all, 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,WEB,weblink IERS science background, IERS, Frankfurt am Main, 2013, August 6, 2016, no,weblink August 29, 2016, mdy-all, 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.WEB,weblink Bulletin C 49, Gambis, Danie, January 5, 2015, IERS, Paris, January 5, 2015, no,weblink May 30, 2015, mdy-all, WEB,weblink 2015 is getting an extra second and that's a bit of a problem for the internet, James Vincent, The Verge, January 7, 2015, no,weblink" title="web.archive.org/web/20170317175523weblink">weblink March 17, 2017, mdy-all, {{Obsolete source|reason=Both of these sources are for the 26th leap second of 2015, not the 27th of 2016.|date=January 2017}}

History

(File:Leapsecond.ut1-utc.svg|thumb|right|300px|Graph showing the difference between UT1 and UTC. Vertical segments correspond to leap seconds.)About 140 AD, Ptolemy, the Alexandrian astronomer, sexagesimally subdivided both the mean solar day and the true solar day to at least six places after the sexagesimal point, and he used simple fractions of both the equinoctial hour and the seasonal hour, none of which resemble the modern second.BOOK, Ptolemy, Ptolemy, G. J. Toomer, Toomer, G. J., Ptolemy's Alemagest, 1998, Princeton University Press, Princeton, New Jersey, 978-0-691-00260-6, 6–7, 23, 211–216, Muslim scholars, including al-Biruni in 1000, subdivided the mean solar day into 24 equinoctial hours, each of which was subdivided sexagesimally, that is into the units of minute, second, third, fourth and fifth, creating the modern second as {{nowrap|1= {{frac|1|60}} of {{frac|1|60}} of {{frac|1|24}} = {{frac|1|86,400}}}} of the mean solar day in the process.BOOK, al-Biruni, al-Biruni, Sachau, C. Edward, The chronology of ancient nations: an English version of the Arabic text of the Athâr-ul-Bâkiya of Albîrûnî, or "Vestiges of the Past",weblink 1879, Oriental Translation Fund of Great Britain & Ireland, 141–149, 158, 408, 410, no,weblink November 14, 2017, mdy-all, Used for mean new moons, both in Hebrew calendar cycles and in equivalent astronomical cycles. With this definition, the second was proposed in 1874 as the base unit of time in the CGS system of units.BOOK, Illustrations of the centimetre-gramme-second (C.G.S.) system of units,weblink 1875, Taylor and Francis, 83, J. D., Everett, Joseph David Everett, no,weblink November 14, 2017, mdy-all, Soon afterwards Simon Newcomb and others discovered that Earth's rotation period varied irregularly,JOURNAL, Pearce, J. A., 1928, The Variability of the Rotation of the Earth, Journal of the Royal Astronomical Society of Canada, 22, 145–147, 1928JRASC..22..145P, so in 1952, the International Astronomical Union (IAU) defined the second as a fraction of the sidereal year. Because the tropical year was considered more fundamental than the sidereal year, in 1955, the IAU redefined the second as the fraction {{frac|1|31,556,925.975}} of the 1900.0 mean tropical year. In 1956, a slightly more precise value of {{frac|1|31,556,925.9747}} was adopted for the definition of the second by the International Committee for Weights and Measures, and in 1960 by the General Conference on Weights and Measures, becoming a part of the International System of Units (SI).BOOK, Explanatory Supplement to the Astronomical Almanac,weblink 1992, University Science Books, Mill Valley, California, 0-935702-68-7, 79–80, P. Kenneth, Seidelmann, no,weblink November 14, 2017, mdy-all, Eventually, this definition too was found to be inadequate for precise time measurements, so in 1967, the SI second was again redefined as 9,192,631,770 periods of the radiation emitted by a caesium-133 atom in the transition between the two hyperfine levels of its ground state.WEB, Leap Seconds, Time Service Department, United States Naval Observatory,weblink December 27, 2008, no,weblink" title="web.archive.org/web/20120207234121weblink">weblink February 7, 2012, mdy-all, That value agreed to 1 part in 1010 with the astronomical (ephemeris) second then in use.Wm Markowitz (1988) 'Comparisons of ET (Solar), ET (Lunar), UT and TDT', in (eds.) A K Babcock & G A Wilkins, 'The Earth's Rotation and Reference Frames for Geodesy and Geophysics', IAU Symposia #128 (1988), at pp 413–418. It was also close to {{frac|1|86,400}} of the mean solar day as averaged between years 1750 and 1892.However, for the past several centuries, the length of the mean solar day has been increasing by about 1.4–1.7 ms per century, depending on the averaging time.DD McCarthy and AK Babcock (1986), "The Length of the Day Since 1658", Phys. Earth Planet Inter., No. 44, pp. 281-292RA Nelson, DD McCarthy, S Malys, J Levine, B Guinot, HF Fliegel, RL Beard, and TR Bartholomew, (2001) "The Leap Second: its History and Possible Future" (2001), Metrologia 38, pp 509-529JOURNAL, Stephenson, F.R., Morrison, L.V., 1995, Long-term fluctuations in the Earth's rotation: 700 BC to AD 1990, 1995RSPTA.351..165S, Philosophical Transactions of the Royal Society of London A, 351, 1695, 165–202, 10.1098/rsta.1995.0028, By 1961, the mean solar day was already a millisecond or two longer than {{formatnum:86400}} SI seconds.JOURNAL, McCarthy, D D, Hackman, C, Nelson, R A, 2008, The Physical Basis of the Leap Second,weblink Astronomical Journal, 136, 5, 1906–1908, 10.1088/0004-6256/136/5/1906, 2008AJ....136.1906M, Therefore, time standards that change the date after precisely {{formatnum:86400}} SI seconds, such as the International Atomic Time (TAI), will get increasingly ahead of time standards tied to the mean solar day, such as Greenwich Mean Time (GMT).When the Coordinated Universal Time standard was instituted in 1961, based on atomic clocks, it was felt necessary to maintain agreement with the GMT time of day, which, until then, had been the reference for broadcast time services. Thus, from 1961 to 1971, the rate of (some) atomic clocks was constantly slowed to remain synchronised with GMT. During that period, therefore, the "seconds" of broadcast services were actually slightly longer than the SI second and closer to the GMT seconds.In 1972, the leap-second system was introduced so that the broadcast UTC seconds could be made exactly equal to the standard SI second, while still maintaining the UTC time of day and changes of UTC date synchronized with those of UT1 (the solar time standard that superseded GMT). By then, the UTC clock was already 10 seconds behind TAI, which had been synchronized with UT1 in 1958, but had been counting true SI seconds since then. After 1972, both clocks have been ticking in SI seconds, so the difference between their readouts at any time is 10 seconds plus the total number of leap seconds that have been applied to UTC (37 seconds {{as of|January 2019|lc=on}}).">

Insertion of leap seconds{| class"wikitable" style"float:right; margin-right: 0; margin-left: 1em; text-align: center;"|+ Announced leap seconds to date

! Year !! Jun 30 !! Dec 31
! 1972
+1 bgcolor="lime"| +1
! 1973
+1
! 1974
+1
! 1975
+1
! 1976
+1
! 1977
+1
! 1978
+1
! 1979
+1
| 1980
| 0
! 1981
+1 0
! 1982
+1 0
! 1983
+1 0
| 1984
| 0
! 1985
+1 0
| 1986
| 0
! 1987
+1
| 1988
| 0
! 1989
+1
! 1990
+1
| 1991
| 0
! 1992
+1 0
! 1993
+1 0
! 1994
+1 0
! 1995
+1
| 1996
| 0
! 1997
+1 0
! 1998
+1
| 1999
| 0
| 2000
| 0
| 2001
| 0
| 2002
| 0
| 2003
| 0
| 2004
| 0
! 2005
+1
| 2006
| 0
| 2007
| 0
! 2008
+1
| 2009
| 0
| 2010
| 0
| 2011
| 0
! 2012
+1 0
| 2013
| 0
| 2014
| 0
! 2015
+1 0
! 2016
+1
| 2017
| 0
| 2018
| 0
| 2019
|
! Year !! Jun 30 !! Dec 31
!rowspan="2"| Total
| 16
27
!colspan="3"| Current TAI − UTC
37
The scheduling of leap seconds was initially delegated to the
Bureau International de l'Heure (BIH), but passed to the International Earth Rotation and Reference Systems Service (IERS) on January 1, 1988. IERS usually decides to apply a leap second whenever the difference between UTC and UT1 approaches 0.6 s, in order to keep the difference between UTC and UT1 from exceeding {{nowrap| 0.9 s.}}The UTC standard allows leap seconds to be applied at the end of any UTC month, with first preference to June and December and second preference to March and September. {{As of|January 2017}}, all of them have been inserted at the end of either June 30 or December 31. IERS publishes announcements every six months, whether leap seconds are to occur or not, in its "Bulletin C". Such announcements are typically published well in advance of each possible leap second date – usually in early January for June 30 and in early July for December 31.WEB, Gambis, Daniel, Bulletin C 36, International Earth Rotation and Reference Systems Service, IERS EOP PC, Paris Observatory, Observatoire de Paris, Paris, July 4, 2008,weblink April 18, 2010, no,weblink" title="web.archive.org/web/20091006221408weblink">weblink October 6, 2009, mdy-all, WEB,weblink 2008 Will Be Just a Second Longer, Andrea Thompson, December 29, 2008, Live Science, December 8, 2008, no,weblink" title="web.archive.org/web/20081212222023weblink">weblink December 12, 2008, mdy-all, Some time signal broadcasts give voice announcements of an impending leap second.Between 1972 and 2018, a leap second has been inserted about every 20 months, on average. However, the spacing is quite irregular and apparently increasing: there were no leap seconds in the seven-year interval between January 1, 1999 and December 31, 2005, but there were nine leap seconds in the eight years 1972–1979.Unlike leap days, UTC leap seconds occur simultaneously worldwide; for example, the leap second on December 31, 2005 23:59:60 UTC was December 31, 2005 18:59:60 (6:59:60 p.m.) in U.S. Eastern Standard Time and January 1, 2006 08:59:60 (a.m.) in Japan Standard Time.Not all clocks implement leap seconds in the same manner as UTC. Leap seconds in Unix time are commonly implemented by repeating the last second of the day. Network Time Protocol freezes time during the leap second. Other experimental schemes smear time in the vicinity of a leap second.{{citation|rfc=7164|author=Kevin Gross|title=RTP and Leap Seconds|date=March 2014}}

Slowing rotation of the Earth

(File:Deviation of day length from SI day.svg|thumb|left|Deviation of day length from SI based day)Leap seconds are irregularly spaced because the Earth's rotation speed changes irregularly. Indeed, the Earth's rotation is quite unpredictable in the long term, which explains why leap seconds are announced only six months in advance.A mathematical model of the variations in the length of the solar day was developed by F. R. Stephenson and L. V. Morrison, based on records of eclipses for the period 700 BC to 1623 AD, telescopic observations of occultations for the period 1623 until 1967 and atomic clocks thereafter. The model shows a steady increase of the mean solar day by {{nowrap|1.70 ms}} {{nowrap|(± 0.05 ms)}} per century, plus a periodic shift of about 4 ms amplitude and period of about 1,500 yr. Over the last few centuries, the periodic component reduced the rate of lengthening of the mean solar day to about {{nowrap|1.4 ms}} per century.WEB,weblink Extrapolations of the difference (TI - UT1), ucolick.org, June 8, 2011, Steve Allen, February 29, 2016, no,weblink" title="web.archive.org/web/20160304193859weblink">weblink March 4, 2016, mdy-all, The main reason for the slowing down of the Earth's rotation is tidal friction, which alone would lengthen the day by 2.3 ms/century. Other contributing factors are the movement of the Earth's crust relative to its core, changes in mantle convection, and any other events or processes that cause a significant redistribution of mass. These processes change the Earth's moment of inertia, affecting the rate of rotation due to conservation of angular momentum. Some of these redistributions increase Earth's rotational speed, shorten the solar day and oppose tidal friction. For example, glacial rebound shortens the solar day by 0.6 ms/century and the 2004 Indian Ocean earthquake is thought to have shortened it by 2.68 microseconds.WEB, Cook-Anderson, Gretchen, Beasley, Dolores,weblink NASA Details Earthquake Effects on the Earth, National Aeronautics and Space Administration (press release)., January 10, 2005, no,weblink" title="web.archive.org/web/20110127163105weblink">weblink January 27, 2011, mdy-all, It is evident from the figure that the Earth's rotation has slowed at a decreasing rate since the initiation of the current system in 1971, and the rate of leap second insertions has therefore been decreasing.

Proposal to abolish leap seconds

{{anchor|Proposal to abolish leap seconds}}The utility of leap seconds is disputed. Greenwich is the historical reference not only for longitude (Greenwich meridian) but also for Universal Time (UT1) based on Earth's rotation. While the TAI and UT1 time scales are precisely defined, the former by atomic clocks and the latter by astronomical observations, UTC is a compromise, stepping with atomic seconds and periodically reset by a leap second to the astronomical time of Greenwich; the intention is to keep civil time aligned with UT1. However, even at Greenwich, leap seconds do not ensure that the sun culminates exactly at 12:00:00.000 UTC, as noon deviates from it up to 16 minutes over the year (the equation of time). All sundials show an offset to civil time. Professional astronomers do not rely on UTC, but on UT1, which has no leap seconds but a varying offset to UTC expressed in DUT1. Orienting a space telescope such as the Hubble Space Telescope cannot use leap seconds. GPS navigation uses the linear GPS time scale, as a one-second leap would cause a location error of up to {{convert|460|m|nmi|frac=4|abbr=off|sp=us}}. If the difference between solar time at a particular location and local time would matter, users simply need to know the difference of UTC to UT1, DUT1 (which is broadcast), as they need to know the difference of their location to the Greenwich meridian (their longitude). Citizens accept yearly variations of one hour because of daylight saving time, they do not care about second-accurate noon. In Europe, citizens are split whether to adopt permanently summer time or standard time, they do not care about leap seconds.{{cn|date=January 2019}} If the difference between solar noon and local time 12:00 would exceed half an hour (that without leap seconds would occur in some 1,000 years from now), a country could change its time zone to align it with its mean solar day, leap seconds are not needed.
The irregularity and unpredictability of UTC leap seconds is problematic for several areas, especially computing. For example, to compute the elapsed time in seconds between two given UTC past dates requires the consultation of a table of leap seconds, which needs to be updated whenever a new leap second is announced. Moreover, it is not possible to compute accurate time intervals for UTC dates that are more than about six months in the future. Most time distribution systems (SNTP, IRIG-B, PTP) only announce leap seconds at most 12 hours in advance and sometimes only in the last minute. With increasing requirements for accuracy in automation systems and high-speed trading, this raises a number of issues, as a leap second represents a jump often a million times larger than the required accuracy for industry clocks. IEC/IEEE 61850-9-3 solves the problem by using a linear count of seconds, including leap seconds, since a specified epoch. On July 5, 2005, the Head of the Earth Orientation Center of the IERS sent a notice to IERS Bulletins C and D subscribers, soliciting comments on a U.S. proposal before the ITU-R Study Group 7's WP7-A to eliminate leap seconds from the UTC broadcast standard before 2008 (the ITU-R is responsible for the definition of UTC).{{efn|The Wall Street Journal noted that the proposal was considered by a U.S. official at the time to be a "private matter internal to the ITU."NEWS,weblink Why the U.S. Wants To End the Link Between Time and Sun, The Wall Street Journal, 2017-10-31, no,weblink November 7, 2017, mdy-all, }} It was expected to be considered in November 2005, but the discussion has since been postponed.NEWS,weblink Leap second talks are postponed, BBC News, 2017-10-31, no,weblink" title="web.archive.org/web/20171107031708weblink">weblink November 7, 2017, mdy-all, Under the proposal, leap seconds would be technically replaced by leap hours as an attempt to satisfy the legal requirements of several ITU-R member nations that civil time be astronomically tied to the Sun.A number of objections to the proposal have been raised. Dr. P. Kenneth Seidelmann, editor of the Explanatory Supplement to the Astronomical Almanac, wrote a letter lamenting the lack of consistent public information about the proposal and adequate justification.WEB,weblink UTC redefinition or change, Kenneth Seidelmann, yes,weblink" title="web.archive.org/web/20060110223323weblink">weblink 2006-01-10, Steve Allen of the University of California, Santa Cruz cited what he claimed to be the large impact on astronomers in a Science News article.MAGAZINE, Cowen, Ron., April 22, 2006,weblink To Leap or Not to Leap: Scientists debate a timely issue, 2006-05-19, no,weblink" title="web.archive.org/web/20060502104629weblink">weblink May 2, 2006, mdy-all, Science News, He has an extensive online siteWEB,weblink UTC might be redefined without Leap Seconds, Steve Allen, 2017-10-31, no,weblink" title="web.archive.org/web/20170603170203weblink">weblink June 3, 2017, mdy-all, devoted to the issues and the history of leap seconds, including a set of references about the proposal and arguments against it.WEB,weblink nc1985wp7a Proposed US Contribution to ITU-R WP 7A, 2017-10-31, no,weblink" title="web.archive.org/web/20170101164347weblink">weblink January 1, 2017, mdy-all, At the 2014 General Assembly of the International Union of Radio Scientists (URSI), Dr. Demetrios Matsakis, the United States Naval Observatory's Chief Scientist for Time Services, presented the reasoning in favor of the redefinition and rebuttals to the arguments made against it.WEB,weblink Comments on the Debate over the Proposal to Redefine UTC, Demetrios Matsakis, August 18, 2014, 2017-10-31, no,weblink" title="web.archive.org/web/20170208050335weblink">weblink February 8, 2017, mdy-all, He stressed the practical inability of software programmers to allow for the fact that leap seconds make time appear to go backwards, particularly when most of them do not even know that leap seconds exist. The possibility of leap seconds being a hazard to navigation was presented, as well as the observed effects on commerce.The United States formulated its position on this matter based upon the advice of the National Telecommunications and Information AdministrationWEB,weblink U.S. Proposals, National Telecommunications and Information Administration, no,weblink" title="web.archive.org/web/20140729083221weblink">weblink July 29, 2014, mdy-all, and the Federal Communications Commission (FCC), which solicited comments from the general public.WEB,weblink FCC Seeks Comment On Recommendations Approved By The Advisory Committee For The 2015 World Radiocommunication Conference, Federal Communications Commission, January 28, 2014, no,weblink July 29, 2014, mdy-all, This position is in favor of the redefinition.WEB,weblink Preliminary Views and Proposals Regarding WRC-15 Agenda Items, Organization of American States, PPT, no,weblink" title="web.archive.org/web/20140729090447weblink">weblink July 29, 2014, mdy-all, {{efn|The FCC has posted its received comments, which can be found using their search engine for proceeding 04-286 and limiting the "received period" to those between January 27 and February 18, 2014 inclusive.WEB,weblink Search for Filings Results, fcc.gov, no,weblink" title="web.archive.org/web/20150701090036weblink">weblink July 1, 2015, mdy-all, }}In 2011, Chunhao Han of the Beijing Global Information Center of Application and Exploration said China had not decided what its vote would be in January 2012, but some Chinese scholars consider it important to maintain a link between civil and astronomical time due to Chinese tradition. The 2012 vote was ultimately deferred.Merali, Zeeya. (November 8, 2011). "Time is running out for the leap second". {{webarchive|url=https://web.archive.org/web/20170516230220weblink |date=May 16, 2017 }} Nature News. At an ITU/BIPM-sponsored workshop on the leap second, Dr. Han expressed his personal view in favor of abolishing the leap second,WEB,weblink Conception, Definition and Realization of Time Scale in GNSS, Han, Chunhao, September 19, 2013, no,weblink September 5, 2014, mdy-all, and similar support for the redefinition was again expressed by Dr. Han, along with other Chinese timekeeping scientists, at the URSI General Assembly in 2014.At a special session of the Asia-Pacific Telecommunity Meeting on February 10, 2015, Chunhao Han indicated China was now supporting the elimination of future leap seconds, as were all the other presenting national representatives (from Australia, Japan, and the Republic of Korea). At this meeting, Bruce Warrington (NMI, Australia) and Tsukasa Iwama (NICT, Japan) indicated particular concern for the financial markets due to the leap second occurring in the middle of a workday in their part of the world.{{efn|In addition to publishing the video of the special session,AV MEDIA,weblink Information Session on the WRC-15 agenda item 1.14 – Coordinated Universal Time (UTC), April 15, 2015, YouTube, no,weblink November 18, 2015, mdy-all, the Australian Communications and Media Authority has a transcript of that session and a web page with draft content of the Conference Preparatory Meeting report and solutions for ITU-R WRC-15 Agenda Item 1.14.WEB,weblinkweblink" title="web.archive.org/web/20150908184107weblink">weblink September 8, 2015, WRC-15 Agenda item 1.14: Coordinated Universal Time (UTC), acma.gov.au, }} Subsequent to the CPM15-2 meeting in March/April 2015 the draft gives four methods which the WRC-15 might use to satisfy Resolution 653 from WRC-12.WEB,weblink RESOLUTION 653 (WRC-12) Future of the Coordinated Universal Time time-scale, International Telecommunications Union, no,weblink" title="web.archive.org/web/20150702175848weblink">weblink July 2, 2015, mdy-all, Arguments against the proposal include the unknown expense of such a major change and the fact that universal time will no longer correspond to mean solar time. It is also answered that two timescales that do not follow leap seconds are already available, International Atomic Time (TAI) and Global Positioning System (GPS) time. Computers, for example, could use these and convert to UTC or local civil time as necessary for output. Inexpensive GPS timing receivers are readily available, and the satellite broadcasts include the necessary information to convert GPS time to UTC. It is also easy to convert GPS time to TAI, as TAI is always exactly 19 seconds ahead of GPS time. Examples of systems based on GPS time include the CDMA digital cellular systems IS-95 and CDMA2000. In general, computer systems use UTC and synchronize their clocks using Network Time Protocol (NTP). Systems that cannot tolerate disruptions caused by leap seconds can base their time on TAI and use Precision Time Protocol. However, the BIPM has pointed out that this proliferation of timescales leads to confusion.{{citation|url=http://www.bipm.org/utils/en/pdf/CCTF-strategy-document.pdf|date=July 29, 2013|title=CCTF Strategy Document|publisher=International Bureau of Weights and Measures|pages=19–24|deadurl=no|archiveurl=https://web.archive.org/web/20140728003658weblink|archivedate=July 28, 2014|df=mdy-all}}At the 47th meeting of the Civil Global Positioning System Service Interface Committee in Fort Worth, Texas in September 2007, it was announced that a mailed vote would go out on stopping leap seconds. The plan for the vote was:WEB,weblink November 18, 2007, September 25, 2007, 9, PDF, 47th CGSIC Meeting - Timing Subcommittee, yes,weblink" title="web.archive.org/web/20110614015830weblink">weblink June 14, 2011, mdy-all,
  • April 2008: ITU Working Party 7A will submit to ITU Study Group 7 project recommendation on stopping leap seconds
  • During 2008, Study Group 7 will conduct a vote through mail among member states
  • October 2011: The ITU-R released its status paper, Status of Coordinated Universal Time (UTC) study in ITU-R, in preparation for the January 2012 meeting in Geneva; the paper reported that, to date, in response to the UN agency's 2010 and 2011 web based surveys requesting input on the topic, it had received 16 responses from the 192 Member States with "13 being in favor of change, 3 being contrary."JOURNAL, WP7D - Status of Coordinated Universal Time (UTC) study in ITU-R, International Telecommunication Union – Radiocommunication Sector (ITU-R) Release, October 4, 2011, Pg.2 (Pgs.2),weblink Word 2007, October 24, 2011, To date, the BR received replies from 16 different Member States for the latest survey (out of a total of 192 Member States, 55 of which participate in the formation of UTC) - 13 being in favor of the change, 3 being contrary., no,weblink" title="web.archive.org/web/20140323233615weblink">weblink March 23, 2014, mdy-all,
  • January 2012: The ITU makes a decision.
In January 2012, rather than decide yes or no per this plan, the ITU decided to postpone a decision on leap seconds to the World Radiocommunication Conference in November 2015. At this conference, it was again decided to continue using leap seconds, pending further study and consideration at the next conference in 2023.WEB, Coordinated Universal Time (UTC) to retain 'leap second', International Telecommunications Union, November 19, 2015, January 12, 2016,weblink no,weblink January 29, 2016, mdy-all, In October 2014, Dr. Włodzimierz Lewandowski, chair of the timing subcommittee of the Civil GPS Interface Service Committee and a member of the ESA Navigation Program Board, presented a CGSIC-endorsed resolution to the ITU that supported the redefinition and described leap seconds as a "hazard to navigation".WEB,weblink CGSIC opinion on the redefinition of UTC now under consideration by the International Telecommunications Union (ITU), no,weblink" title="web.archive.org/web/20141023113730weblink">weblink October 23, 2014, mdy-all, Some of the objections to the proposed change have been answered by its opponents. For example, Dr. Felicitas Arias, who, as Director of the International Bureau of Weights and Measures (BIPM)'s Time, Frequency, and Gravimetry Department, is responsible for generating UTC, noted in a press release that the drift of about one minute every 60–90 years could be compared to the 16-minute annual variation between true solar time and mean solar time, the one hour offset by use of daylight time, and the several-hours offset in certain geographically extra-large time zones.PRESS RELEASE,weblink The proposed redefinition of Coordinated Universal Time, UTC, BIPM, October 13, 2011, no,weblink" title="web.archive.org/web/20150118133939weblink">weblink January 18, 2015, mdy-all,

Examples of problems associated with the leap second

While the textual representation of leap seconds is defined by BIMP as "23:59:60", some computers derive this human-readable representation from a binary counter giving the number of seconds elapsed since an epoch, for instance since 1970-01-01 00:00:00 in Unix machines. This counter has no indicator that a leap second is occurring. Some computers, in particular Linux, assign to the leap second the number of the preceding 23:59:59 second (9-9-0 sequence), while other computers assign to the leap second the counter value of the next 00:00:00 second (9-0-0 sequence). The BIMP definition in Bulletin C52 calls for the 9-0-0 sequence. Since there is no standard governing the sequence, the time stamp values can vary by one second. Entering "2016-12-31 23:59:60 in a POSIX converter will fail and XML will reject such date as "invalid time". This may explain many flaws in time-critical systems that occur when exchanging time-stamped values. A number of organizations reported problems caused by flawed software following the June 30, 2012 leap second. Among the sites which reported problems were Reddit (Apache Cassandra), Mozilla (Hadoop),WEB,weblink ‘Leap Second’ Bug Wreaks Havoc Across Web, Wired, July 1, 2012, no,weblink" title="web.archive.org/web/20140328124116weblink">weblink March 28, 2014, mdy-all, Qantas,WEB,weblink 'Leap second crashes Qantas and leaves passengers stranded', News Limited, July 1, 2012, no,weblink" title="web.archive.org/web/20120701162924weblink">weblink July 1, 2012, mdy-all, and various sites running Linux.WEB,weblink Anyone else experiencing high rates of Linux server crashes during a leap second day?, Serverfault.com, no,weblink" title="web.archive.org/web/20120709110353weblink">weblink July 9, 2012, mdy-all, Older versions of Motorola Oncore VP, UT, GT, and M12 GPS receivers had a software bug that would cause a single timestamp to be off by a day if no leap second was scheduled for 256 weeks. On November 28, 2003, this happened. At midnight, the receivers with this firmware reported November 29, 2003 for one second and then reverted to November 28, 2003.WEB,weblink 256-Week Leap Second Bug, July 2, 2013, no,weblink" title="web.archive.org/web/20160304002759weblink">weblink March 4, 2016, mdy-all, WEB,weblink Motorola Oncore receivers and Leap Second bug, July 2, 2013, no,weblink" title="web.archive.org/web/20130118233907weblink">weblink January 18, 2013, mdy-all, Older Trimble GPS receivers had a software flaw that would insert a leap second immediately after the GPS constellation started broadcasting the next leap second insertion time (some months in advance of the actual leap second), rather than waiting for the next leap second to happen. This left the receiver's time off by a second in the interim.WEB,weblink Leap-second problem with older GPS receivers, November 19, 2014, no,weblink" title="web.archive.org/web/20141129055128weblink">weblink November 29, 2014, mdy-all, WEB,weblink How Leap Seconds Can Interfere with GNSS Receivers, May 13, 2015, no,weblink" title="web.archive.org/web/20160306014132weblink">weblink March 6, 2016, mdy-all, Older Datum Tymeserve 2100 GPS receivers and Symmetricom Tymeserve 2100 receivers also have a similar flaw to that of the older Trimble GPS receivers, with the time being off by one second. The advance announcement of the leap second is applied as soon as the message is received, instead of waiting for the correct date. A workaround has been described and tested, but if the GPS system rebroadcasts the announcement, or the unit is powered off, the problem will occur again.MAILING LIST,weblink Symmetricom TymServe 2100-GPS currently fails with GPS offset, time-nuts, no,weblink" title="web.archive.org/web/20150217202809weblink">weblink February 17, 2015, mdy-all, On January 21, 2015, several models of GPS receivers implemented the leap second as soon as the announcement was broadcast by GPS, instead of waiting until the implementation date of June 30.WEB,weblink Leap Second Implementation Confuses Some Receivers, GPS World, February 2, 2015, no,weblink" title="web.archive.org/web/20150505073219weblink">weblink May 5, 2015, mdy-all, The NTP packet includes a leap second flag, which informs the user that a leap second is imminent. This, among other things, allows the user to distinguish between a bad measurement that should be ignored and a genuine leap second that should be followed. It has been reported that never, since the monitoring began in 2008 and whether or not a leap second should be inserted, have all NTP servers correctly set their flags on a December 31 or June 30.Dr. David Malone, WEB,weblink Archived copy, 2014-07-15, no,weblink" title="web.archive.org/web/20140722122446weblink">weblink July 22, 2014, mdy-all, CONFERENCE, David, Malone, 2016, The Leap Second Behaviour of NTP Servers, Proc.Traffic Monitoring and Analysis workshop,weblink October 23, 2016, no,weblink" title="web.archive.org/web/20161023201948weblink">weblink October 23, 2016, mdy-all, This is one reason many NTP servers broadcast the wrong time for up to a day after a leap second insertion,WEB,weblink NTP Events, satsignal.eu, no,weblink" title="web.archive.org/web/20140718204646weblink">weblink July 18, 2014, mdy-all, and it has been suggested that hackers have exploited this vulnerability.WEB,weblink WARNING: someone's faking a leap second tonight, ntp:questions, no,weblink" title="web.archive.org/web/20140727203655weblink">weblink July 27, 2014, mdy-all, NEWSGROUP,weblink WARNING: someone's faking a leap second tonight, Marco Marongiu, July 31, 2012, comp.protocols.time.ntp, no,weblinkweblink January 22, 2011, mdy-all, Detailed studies of the leap seconds of 2015 and 2016 show that, even for the Stratum-1 servers which anchor the NTP server network, errors both in leap second flags and the server clocks themselves are widespread, and can be severe.Cao, Yi, Veitch, Darryl, ‘’Network Timing, Weathering the 2016 Leap Second’’. IEEE Infocom 2016, Honolulu, USA, April 15-19, 2018. Veitch, Darryl, Vijayalayan, Kanthaiah, ‘’Network Timing and the 2015 Leap Second’’. Proc. of PAM 2016, Heraklion, Crete, Greece, March 31 --April 1, 2016.Four different brands of marketed navigational receivers that use data from GPS or Galileo along with the Chinese BeiDou satellites, and even some receivers that use BeiDou satellites alone, were found to implement leap seconds one day early.WEB,weblink BeiDou Numbering Presents Leap-Second Issue, GPS World, March 3, 2015, no,weblink" title="web.archive.org/web/20150306020907weblink">weblink March 6, 2015, mdy-all, This was traced to the fact that BeiDou numbers the days of the week from 0 to 6, while GPS and Galileo number them from 1 to 7. The problem was found to exist in commercial simulators that are used by manufacturers to test their equipment.The effect of leap seconds on the commercial sector has been described as "a nightmare".WEB, Sellwood, Dick, Just A Second,weblink www.eejournal.com, Electronic Engineering Journal, September 27, 2015, no,weblink" title="web.archive.org/web/20150928235116weblink">weblink September 28, 2015, mdy-all, Because financial markets are vulnerable to both technical and legal leap second problems, the Intercontinental Exchange, parent body to 7 clearing houses and 11 stock exchanges including the New York Stock Exchange, ceased operations for 61 minutes at the time of the June 30, 2015 leap second.WEB,weblink ICE Market Update - Leap Second Impact, Intercontinental Exchange, yes,weblink May 5, 2015, mdy-all, Despite the publicity given to the 2015 leap second, Internet network failures occurred due to the vulnerability of at least one class of router.NEWS, Shulman, Eden, Beta Boston,weblink September 27, 2015, Boston Globe, no,weblink" title="web.archive.org/web/20150929011652weblink">weblink September 29, 2015, mdy-all, Also, interruptions of around 40 minutes duration occurred with Twitter, Instagram, Pinterest, Netflix, Amazon, and Apple's music streaming series Beats 1.NEWS,weblink Leap Second confuses Twitter and Android, Sarah Knapton, July 1, 2015, The Daily Telegraph, no,weblink" title="web.archive.org/web/20151006002636weblink">weblink October 6, 2015, mdy-all, Several versions of the Cisco Systems NEXUS 5000 Series Operating System NX-OS (versions 5.0, 5.1, 5.2) are affected.WEB,weblink Cisco Bug: CSCub38654 - N5K: Switch hang/lock up may occur due to Leap second update, Cisco, July 24, 2015, no,weblink March 8, 2016, mdy-all, The 2015 leap second also affected the Altea airlines reservation system used by Qantas and Virgin Australia.WEB,weblink Power cut crashes Delta's worldwide flight update systems, 11:33, August 8, 2016 at, tweet_btn(), Gavin Clarke, January 3, 2017, no,weblink" title="web.archive.org/web/20170104002416weblink">weblink January 4, 2017, mdy-all, Cloudflare was affected by the 2016 leap second. Its DNS resolver implementation calculated a negative number when subtracting two timestamps obtained from the Go programming language's time.Now() function, which then used only a real-time clock source.WEB,weblink How and why the leap second affected Cloudflare DNS, Cloudflare, January 1, 2017, no,weblink January 2, 2017, mdy-all, This could have been avoided by using a monotonic clock source, which has since been added to Go 1.9.WEB, #12914 runtime: time: expose monotonic clock source,weblink GitHub, January 5, 2017, no,weblink March 20, 2017, mdy-all, There were concerns that farming equipment using GPS during harvests occurring on December 31, 2016 would be affected by the 2016 leap second.NEWS,weblink Got a second — the world time needs it, December 30, 2016, ABC Rural, en-AU, January 3, 2017, no,weblink" title="web.archive.org/web/20170102192801weblink">weblink January 2, 2017, mdy-all,

Workarounds for leap second problems

The most obvious workaround is to use the TAI scale for all operational purposes and convert to UTC for human-readable text. UTC can always be derived from TAI with a suitable table of leap seconds. The SMTPE video/audio industry standards body selected TAI for deriving time stamps of media.weblink, IEEE SMPTE IEC/IEEE 60802 (Time sensitive networks) specifies TAI for all operations. Grid automation is planning to switch to TAI for global distribution of events in electrical grids. Instead of inserting a leap second at the end of the day, Google servers implement a "leap smear", extending seconds slightly over a time window prior to the leap second.WEB,weblink Time, technology and leaping seconds, September 15, 2011, Christopher Pascoe, Google, July 2, 2012, no,weblink" title="web.archive.org/web/20120702030514weblink">weblink July 2, 2012, mdy-all, Amazon followed a similar, but slightly different, pattern for the introduction of the June 30, 2015 leap second,WEB,weblink Look Before You Leap – The Coming Leap Second and AWS (Updated), Jeff Barr, May 18, 2015, Amazon Web Services, no,weblink May 19, 2015, mdy-all, leading to another case of the proliferation of timescales. They later released an NTP service for EC2 instances which performs leap smearing.WEB,weblink Keeping Time With Amazon Time Sync Service, Randall Hunt, November 29, 2017, Amazon Web Services, March 8, 2018, no, mdy-all, It has been proposed that media clients using the Real-time Transport Protocol inhibit generation or use of NTP timestamps during the leap second and the second preceding it.WEB,weblink RTP and Leap Seconds, Kevin Gross, June 21, 2012, Internet Engineering Task Force, July 2, 2012, no,weblink" title="web.archive.org/web/20121216211331weblink">weblink December 16, 2012, mdy-all, NIST has established a special NTP time server to deliver UT1 instead of UTC.WEB,weblink Archived copy, 2017-07-30, no,weblink" title="web.archive.org/web/20160823000152weblink">weblink August 23, 2016, mdy-all, Such a server would be particularly useful in the event the ITU resolution passes and leap seconds are no longer inserted.CONFERENCE, Patrick, Wallace, 2003, The UTC Problem and its Solution, Proceedings of Colloquium on the UTC Time Scale, Torino,weblink no,weblink" title="web.archive.org/web/20150118134330weblink">weblink January 18, 2015, mdy-all, Those astronomical observatories and other users that require UT1 could run off UT1 – although in many cases these users already download UT1-UTC from the IERS, and apply corrections in software.WEB, Brian, Luzum, The Role of the IERS in the Leap Second, BIPM/ITU Workshop on the Future of the International Time Scale, 2013,weblink no,weblink July 15, 2014, mdy-all,

See also

Notes

{{notelist}}

References

{{reflist|30em}}

Further reading

  • Ahuja, Anjana (October 30, 2005). "Savouring the last leap second in history". New Straits Times, p. F10.
  • Grossman, Wendy M. (November 2005). "Wait a Second". Scientific American, pp. 12–13.
  • JOURNAL, Finkleman, David, etal, The Future of Time: UTC and the Leap Second, 1106.3141, American Scientist, 99, 4, 312–319, 10.1511/2011.91.1,
  • JOURNAL, Kamp, Poul-Henning, 2011, The One-Second War, Communications of the ACM, 54, 5, 44–48, 10.1145/1941487.1941505,
  • McCarthy, Dennis D. & Seidelmann, P. Kenneth. (2009). TIME From Earth Rotation to Atomic Physics. Weinheim: Wiley-VCH.

External links

{{commons category|Leap second}} {{Time Topics}}{{Time measurement and standards}}{{New Year}}

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