Leap second

A leap second is a one-second adjustment that is occasionally applied to civil time Coordinated Universal Time (UTC) to keep it close to the mean solar time at Greenwich, in spite of the Earth's rotation slowdown and irregularities. UTC was introduced on January 1, 1972, initially with a 10 second lag behind International Atomic Time (TAI). Since that date, 27 leap seconds have been inserted, the most recent on December 31, 2016 at 23:59:60 UTC, so in 2018, UTC lags behind TAI by an offset of 37 seconds.[1]

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 (86,400 SI seconds). UTC would step ahead of solar time and need adjustment even if the Earth's rotation remained constant in the future. Therefore, if the UTC day were defined as precisely 86,400 SI seconds, the UTC time-of-day would slowly drift apart from that of solar-based standards, such as Greenwich Mean Time (GMT) and its successor UT1. The point on the Earth's equator where the sun culminates at 12:00:00 UTC would wander to the East by some 300 m each year. The leap second compensates for this drift, by occasionally scheduling a UTC day with 86,401 or (in principle) 86,399 SI seconds.

When it occurs, a positive leap second is inserted between second 23:59:59 of a chosen UTC calendar date and second 00:00:00 of the following date. The definition of UTC states that the last day of December and June are preferred, with the last day of March or September as second preference, and the last day of any other month as third preference.[2] 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,[3] 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.[4][5]

Leapsecond2016
Screenshot of the UTC clock from time.gov 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.

History

Leapsecond.ut1-utc
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.[6] 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 160 of ​160 of ​124 = ​186,400 of the mean solar day in the process.[7] With this definition, the second was proposed in 1874 as the base unit of time in the CGS system of units.[8] Soon afterwards Simon Newcomb and others discovered that Earth's rotation period varied irregularly,[9] 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 ​131,556,925.975 of the 1900.0 mean tropical year. In 1956, a slightly more precise value of ​131,556,925.9747 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).[10]

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.[11] That value agreed to 1 part in 1010 with the astronomical (ephemeris) second then in use.[12] It was also close to ​186,400 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.[13][14][15] By 1961, the mean solar day was already a millisecond or two longer than 86,400 SI seconds.[16] Therefore, time standards that change the date after precisely 86,400 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).[11] 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).

Insertion of leap seconds

Announced leap seconds to date
Year Jun 30 Dec 31
1972 +1 +1
1973 0 +1
1974 0 +1
1975 0 +1
1976 0 +1
1977 0 +1
1978 0 +1
1979 0 +1
1980 0 0
1981 +1 0
1982 +1 0
1983 +1 0
1984 0 0
1985 +1 0
1986 0 0
1987 0 +1
1988 0 0
1989 0 +1
1990 0 +1
1991 0 0
1992 +1 0
1993 +1 0
1994 +1 0
1995 0 +1
1996 0 0
1997 +1 0
1998 0 +1
1999 0 0
2000 0 0
2001 0 0
2002 0 0
2003 0 0
2004 0 0
2005 0 +1
2006 0 0
2007 0 0
2008 0 +1
2009 0 0
2010 0 0
2011 0 0
2012 +1 0
2013 0 0
2014 0 0
2015 +1 0
2016 0 +1
2017 0 0
2018 0 0
2019 0
Year Jun 30 Dec 31
Total 11 16
27
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 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.[17][18] 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.[19]

Slowing rotation of the Earth

Deviation of day length from SI day
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,[15] 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 1.70 ms (± 0.05 ms) per century, plus a periodic shift of about 4 ms amplitude and period of about 1,500 yr.[15] Over the last few centuries, the periodic component reduced the rate of lengthening of the mean solar day to about 1.4 ms per century.[20]

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.[15] 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.[21] 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

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 460 meters (14 nautical mile). 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. 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).[a] It was expected to be considered in November 2005, but the discussion has since been postponed.[23] 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.[24] 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.[25] He has an extensive online site[26] devoted to the issues and the history of leap seconds, including a set of references about the proposal and arguments against it.[27]

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.[28] 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 Administration[29] and the Federal Communications Commission (FCC), which solicited comments from the general public.[30] This position is in favor of the redefinition.[31][b]

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.[33] At an ITU/BIPM-sponsored workshop on the leap second, Dr. Han expressed his personal view in favor of abolishing the leap second,[34] 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.[c] 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.[37]

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.[38]

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:[39]

  • 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."[40]
  • 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.[41]

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".[42]

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.[43]

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 [1] 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),[44] Qantas,[45] and various sites running Linux.[46]

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.[47][48]

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.[49][50]

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.[51]

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.[52]

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.[53][54] This is one reason many NTP servers broadcast the wrong time for up to a day after a leap second insertion,[55] and it has been suggested that hackers have exploited this vulnerability.[56][57] 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.[58][59]

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.[60] 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".[61] 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.[62]

Despite the publicity given to the 2015 leap second, Internet network failures occurred due to the vulnerability of at least one class of router.[63] Also, interruptions of around 40 minutes duration occurred with Twitter, Instagram, Pinterest, Netflix, Amazon, and Apple's music streaming series Beats 1.[64]

Several versions of the Cisco Systems NEXUS 5000 Series Operating System NX-OS (versions 5.0, 5.1, 5.2) are affected.[65]

The 2015 leap second also affected the Altea airlines reservation system used by Qantas and Virgin Australia.[66]

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.[67] This could have been avoided by using a monotonic clock source, which has since been added to Go 1.9.[68]

There were concerns that farming equipment using GPS during harvests occurring on December 31, 2016 would be affected by the 2016 leap second.[69]

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.[70] 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.[71] Amazon followed a similar, but slightly different, pattern for the introduction of the June 30, 2015 leap second,[72] leading to another case of the proliferation of timescales. They later released an NTP service for EC2 instances which performs leap smearing.[73]

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.[74]

NIST has established a special NTP time server to deliver UT1 instead of UTC.[75] Such a server would be particularly useful in the event the ITU resolution passes and leap seconds are no longer inserted.[76] 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.[77]

See also

Notes

  1. ^ 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."[22]
  2. ^ 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.[32]
  3. ^ In addition to publishing the video of the special session,[35] 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.[36]

References

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Further reading

External links

Coordinated Universal Time

Coordinated Universal Time (abbreviated to UTC) is the primary time standard by which the world regulates clocks and time. It is within about 1 second of mean solar time at 0° longitude, and is not adjusted for daylight saving time. In some countries where English is spoken, the term Greenwich Mean Time (GMT) is often used as a synonym for UTC and predates UTC by nearly 300 years.The first Coordinated Universal Time was informally adopted on 1 January 1960 and was first officially adopted as CCIR Recommendation 374, Standard-Frequency and Time-Signal Emissions, in 1963, but the official abbreviation of UTC and the official English name of Coordinated Universal Time (along with the French equivalent) were not adopted until 1967.The system has been adjusted several times, including a brief period where time coordination radio signals broadcast both UTC and "Stepped Atomic Time (SAT)" before a new UTC was adopted in 1970 and implemented in 1972. This change also adopted leap seconds to simplify future adjustments. This CCIR Recommendation 460 "stated that (a) carrier frequencies and time intervals should be maintained constant and should correspond to the definition of the SI second; (b) step adjustments, when necessary, should be exactly 1 s to maintain approximate agreement with Universal Time (UT); and (c) standard signals should contain information on the difference between UTC and UT."A number of proposals have been made to replace UTC with a new system that would eliminate leap seconds. A decision whether to remove them altogether has been deferred until 2023.The current version of UTC is defined by International Telecommunications Union Recommendation (ITU-R TF.460-6), Standard-frequency and time-signal emissions, and is based on International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the slowing of the Earth's rotation. Leap seconds are inserted as necessary to keep UTC within 0.9 seconds of the UT1 variant of universal time. See the "Current number of leap seconds" section for the number of leap seconds inserted to date.

DCF77

DCF77 is a German longwave time signal and standard-frequency radio station. It started service as a standard-frequency station on 1 January 1959. In June 1973 date and time information was added. Its primary and backup transmitter are located at 50°0′56″N 9°00′39″E in Mainflingen, about 25 km south-east of Frankfurt am Main, Germany. The transmitter generates a nominal power of 50 kW, of which about 30 to 35 kW can be radiated via a T-antenna.

DCF77 is controlled by the Physikalisch-Technische Bundesanstalt (PTB), Germany's national physics laboratory and transmits in continuous operation (24 hours). It is operated by Media Broadcast GmbH (previously a subsidiary of Deutsche Telekom AG), on behalf of the PTB. With Media Broadcast GmbH, a temporal transmission availability of at least 99.7% per year or under 26.28 hours of annual downtime has been agreed upon. Most service interruptions are short-term disconnections of under two minutes. Longer lasting transmission service interruptions are generally caused by strong winds, freezing rain or snow induced T-antenna movement. This manifests itself in electrical detuning of the antenna resonance circuit and hence a measurable phase modulation of the received signal. When the maladjustment is too large, the transmitter is taken out of service temporarily. In the year 2002, almost 99.95% availability, or just over 4.38 hours of downtime, was realized. The timestamp sent is either in Coordinated Universal Time (UTC)+1 or UTC+2 depending on daylight saving time.The highly accurate 77.5 kHz (approximately 3868.3 m wavelength) carrier signal is generated from local atomic clocks that are linked with the German master clocks at the PTB in Braunschweig. The DCF77 time signal is used for the dissemination of the German national legal time to the public.Radio clocks and watches have been very popular in Europe since the late 1980s and, in mainland Europe, most of them use the DCF77 signal to set their time automatically. Further industrial time-keeping systems at railway stations, in the field of telecommunication and information technology, at radio and TV stations are radio-controlled by DCF77 as well as tariff change-over clocks of energy supply companies and clocks in traffic-light facilities.

Day

A day is approximately the period of time during which the Earth completes one rotation around its axis. A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times.In 1960, the second was redefined in terms of the orbital motion of the Earth in year 1900, and was designated the SI base unit of time. The unit of measurement "day", was redefined as 86,400 SI seconds and symbolized d. In 1967, the second and so the day were redefined by atomic electron transition. A civil day is usually 86,400 seconds, plus or minus a possible leap second in Coordinated Universal Time (UTC), and occasionally plus or minus an hour in those locations that change from or to daylight saving time.Day can be defined as each of the twenty-four-hour periods, reckoned from one midnight to the next, into which a week, month, or year is divided, and corresponding to a rotation of the earth on its axis. However its use depends on its context, for example when people say 'day and night', 'day' will have a different meaning. It will mean the interval of light between two successive nights; the time between sunrise and sunset, in this instance 'day' will mean time of light between one night and the next. However, in order to be clear when using 'day' in that sense, "daytime" should be used to distinguish it from "day" referring to a 24-hour period; this is since daytime usually always means 'the time of the day between sunrise and sunset. The word day may also refer to a day of the week or to a calendar date, as in answer to the question, "On which day?" The life patterns (circadian rhythms) of humans and many other species are related to Earth's solar day and the day-night cycle.

Greenwich Time Signal

The Greenwich Time Signal (GTS), popularly known as the pips, is a series of six short tones broadcast at one-second intervals by many BBC Radio stations. The pips were introduced in 1924 and have been generated by the BBC since 1990 to mark the precise start of each hour. Their utility in calibration is diminishing as digital broadcasting entails time lags.

Hour

An hour (symbol: h; also abbreviated hr.) is a unit of time conventionally reckoned as ​1⁄24 of a day and scientifically reckoned as 3,599–3,601 seconds, depending on conditions.

The hour was initially established in the ancient Near East as a variable measure of ​1⁄12 of the night or daytime. Such seasonal, temporal, or unequal hours varied by season and latitude. The hour was subsequently divided into 60 minutes, each of 60 seconds. Equal or equinoctial hours were taken as ​1⁄24 of the day as measured from noon to noon; the minor seasonal variations of this unit were eventually smoothed by making it ​1⁄24 of the mean solar day. Since this unit was not constant due to long term variations in the Earth's rotation, the hour was finally separated from the Earth's rotation and defined in terms of the atomic or physical second.

In the modern metric system, hours are an accepted unit of time defined as 3,600 atomic seconds. However, on rare occasions an hour may incorporate a positive or negative leap second, making it last 3,599 or 3,601 seconds, in order to keep it within 0.9 seconds of UT1, which is based on measurements of the mean solar day.

Intercalation (timekeeping)

Intercalation or embolism in timekeeping is the insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.

International Atomic Time

International Atomic Time (TAI, from the French name temps atomique international) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. It is the principal realisation of Terrestrial Time (with a fixed offset of epoch). It is also the basis for Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth's surface. As of 31 December 2016, when another leap second was added, TAI is exactly 37 seconds ahead of UTC. The 37 seconds results from the initial difference of 10 seconds at the start of 1972, plus 27 leap seconds in UTC since 1972.

TAI may be reported using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth. Specifically, both Julian Dates and the Gregorian calendar are used. TAI in this form was synchronised with Universal Time at the beginning of 1958, and the two have drifted apart ever since, due to the changing motion of the Earth.

JJY

JJY is the call sign of a low frequency time signal radio station located in Japan.

The station broadcasts from two sites, one on Mount Otakadoya, near Fukushima, and the other on Mount Hagane, located on Kyushu Island. JJY is operated by the National Institute of Information and Communications Technology (NICT), an independent administrative institution affiliated with the Ministry of Internal Affairs and Communications of the Japanese government.

JN53DV

​JN53dv is the Maidenhead grid square of an experimental shortwave time signal station in Italy. It is located in the town of Corsanico-Bargecchia near Massarosa and operated by Italcable

Minute

The minute is a unit of time or angle. As a unit of time, the minute is most of times equal to ​1⁄60 (the first sexagesimal fraction) of an hour, or 60 seconds. In the UTC time standard, a minute on rare occasions has 61 seconds, a consequence of leap seconds (there is a provision to insert a negative leap second, which would result in a 59-second minute, but this has never happened in more than 40 years under this system). As a unit of angle, the minute of arc is equal to ​1⁄60 of a degree, or 60 seconds (of arc). Although not an SI unit for either time or angle, the minute is accepted for use with SI units for both. The SI symbols for minute or minutes are min for time measurement, and the prime symbol after a number, e.g. 5′, for angle measurement. The prime is also sometimes used informally to denote minutes of time.

Network Time Protocol

The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks. In operation since before 1985, NTP is one of the oldest Internet protocols in current use. NTP was designed by David L. Mills of the University of Delaware.

NTP is intended to synchronize all participating computers to within a few milliseconds of Coordinated Universal Time (UTC). It uses the intersection algorithm, a modified version of Marzullo's algorithm, to select accurate time servers and is designed to mitigate the effects of variable network latency. NTP can usually maintain time to within tens of milliseconds over the public Internet, and can achieve better than one millisecond accuracy in local area networks under ideal conditions. Asymmetric routes and network congestion can cause errors of 100 ms or more.The protocol is usually described in terms of a client-server model, but can as easily be used in peer-to-peer relationships where both peers consider the other to be a potential time source. Implementations send and receive timestamps using the User Datagram Protocol (UDP) on port number 123. They can also use broadcasting or multicasting, where clients passively listen to time updates after an initial round-trip calibrating exchange. NTP supplies a warning of any impending leap second adjustment, but no information about local time zones or daylight saving time is transmitted.The current protocol is version 4 (NTPv4), which is a proposed standard as documented in RFC 5905. It is backward compatible with version 3, specified in RFC 1305.

Second

The second is the base unit of time in the International System of Units (SI), commonly understood and historically defined as ​1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each. Mechanical and electric clocks and watches usually have a face with 60 tickmarks representing seconds and minutes, traversed by a second hand and minute hand. Digital clocks and watches often have a two-digit counter that cycles through seconds. The second is also part of several other units of measurement like meters per second for velocity, meters per second per second for acceleration, and per second for frequency.

Although the historical definition of the unit was based on this division of the Earth's rotation cycle, the formal definition in the International System of Units (SI) is a much steadier timekeeper: 1 second is defined to be exactly "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom" (at a temperature of 0 K).

Because the Earth's rotation varies and is also slowing ever so slightly, a leap second is periodically added to clock time to keep clocks in sync with Earth's rotation.

Multiples of seconds are usually counted in hours and minutes. Fractions of a second are usually counted in tenths or hundredths. In scientific work, small fractions of a second are counted in milliseconds (thousandths), microseconds (millionths), nanoseconds (billionths), and sometimes smaller units of a second.

An everyday experience with small fractions of a second is a 1-gigahertz microprocessor which has a cycle time of 1 nanosecond. Camera shutter speeds usually range from ​1⁄60 second to ​1⁄250 second.

Sexagesimal divisions of the day from a calendar based on astronomical observation have existed since the third millennium BC, though they were not seconds as we know them today. Small divisions of time could not be counted back then, so such divisions were figurative. The first timekeepers that could count seconds accurately were pendulum clocks invented in the 17th century. Starting in the 1950s, atomic clocks became better timekeepers than earth's rotation, and they continue to set the standard today.

TDF time signal

TéléDiffusion de France broadcast the TDF time signal, controlled by LNE–SYRTE, from the Allouis longwave transmitter at 162 kHz, with a power of 2 MW.It was also known as FI or France Inter because the signal was formerly best known for broadcasting the France Inter AM signal. This signal ceased at the end of 2016, but the transmitter remains in use for its time signal and other digital signals.

In 1980, the first atomic clock was installed to regulate the carrier frequency. The current time signal is generated by an extremely accurate caesium fountain atomic clock and phase-modulated on the 162 kHz carrier in a way that is inaudible when listening to the France Inter signal using a normal AM receivers. It requires a more complex receiver than the popular DCF77 service, but the much more powerful transmitter (22 to 40 times DCF77's 50 kW) gives it a much greater range of 3,500 km.

The signal is almost continuous but there is a regularly scheduled interruption for maintenance every Tuesday. This used to be from 01:03 to 05:00, but with the cessation of audio signals, it has been moved to 08:00 to 12:00.The signal was formerly 2,000 kW, but has been reduced to 1,500 kW, and tests are in progress of a further reduction to 1,100 kW for cost savings.

Time from NPL (MSF)

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

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

Universal Time

Universal Time (UT) is a time standard based on Earth's rotation. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, England. In fact, the expression "Universal Time" is ambiguous (when accuracy of better than a few seconds is required), as there are several versions of it, the most commonly used being Coordinated Universal Time (UTC) and UT1 (see § Versions). All of these versions of UT, except for UTC, are based on Earth's rotation relative to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time. UTC is based on International Atomic Time, with leap seconds added to keep it within 0.9 second of UT1.

Unix time

Unix time (also known as POSIX time or UNIX Epoch time) is a system for describing a point in time. It is the number of seconds that have elapsed since 00:00:00 Thursday, 1 January 1970, Coordinated Universal Time (UTC), minus leap seconds. Every day is treated as if it contains exactly 86400 seconds, so leap seconds are to be subtracted since the epoch. It is used widely in Unix-like and many other operating systems and file formats. However, Unix time is not a true representation of UTC, as a leap second in UTC shares the same Unix time as the second which came before it. Unix time may be checked on most Unix systems by typing date +%s on the command line.

On systems where Unix time is stored as a signed 32-bit number, the largest value that can be recorded is 2147483647 (231 − 1), which is 03:14:07 Tuesday, 19 January 2038 UTC. The following second, the clock will wrap around to negative 2147483648 (−231), which is 20:45:52 Friday, 13 December 1901 UTC. This is referred to as the Year 2038 problem.

WWVB

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

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

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

WWV (radio station)

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

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

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

World Radiocommunication Conference

World Radiocommunication Conference (WRC) is organized by ITU to review and as necessary, revise the Radio Regulations, the international treaty governing the use of the radio-frequency spectrum and the geostationary-satellite and non-geostationary-satellite orbits. It is held every three to four years. Prior to 1993, it was called the World Administrative Radio Conference (WARC); in 1992, at an Additional Plenipotentiary Conference in Geneva, the ITU was restructured, and later conferences became the WRC.At the 2015 conference (WRC-15), the ITU deferred their decision on whether to abolish the leap second to 2023.The next World Radiocommunication Conference (WRC-19) will take place from 28 October to 22 November 2019 in Sharm el-Sheikh, Egypt.

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