Leap second
Because the Earth's rotational speed varies in response to climatic and geological events,[2] UTC leap seconds are irregularly spaced and unpredictable.
[3][4] This practice has proven disruptive, particularly in the twenty-first century and especially in services that depend on precise timestamping or time-critical process control.
[5] After many years of discussions by different standards bodies, in November 2022, at the 27th General Conference on Weights and Measures, it was decided to abandon the leap second by or before 2035.
[10] Soon afterwards Simon Newcomb and others discovered that Earth's rotation period varied irregularly,[11] so in 1952, the International Astronomical Union (IAU) defined the second as a fraction of the sidereal year.
[12] 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.
This predominantly frequency-shifted rate of UTC was broadcast by MSF, WWV, and CHU among other time stations.
Indeed, the Earth's rotation is quite unpredictable in the long term, which explains why leap seconds are announced only six months in advance.
[17] 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 the conservation of angular momentum.
Some of these redistributions increase Earth's rotational speed, shorten the solar day and oppose tidal friction.
[34] In a 2024 paper published in Nature, Duncan Agnew of the Scripps Institution of Oceanography projects that the water from increasing ice cap melting will migrate to the equator and thus cause the rate of rotation to slow down again.
With increasing requirements for timestamp accuracy in systems such as process automation and high-frequency trading,[35] this raises a number of issues.
Consequently, the long-standing practice of inserting leap seconds is under review by the relevant international standards body.
On 5 July 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).
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.
[39] In an op-ed for Science News, Steve Allen of the University of California, Santa Cruz said that the process has a large impact on astronomers.
The United States formulated its position on this matter based upon the advice of the National Telecommunications and Information Administration[42] and the Federal Communications Commission (FCC), which solicited comments from the general public.
At a special session of the Asia-Pacific Telecommunity meeting on 10 February 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.
In general, computer systems use UTC and synchronize their clocks using Network Time Protocol (NTP).
[51] 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.
[54] In October 2014, 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".
[6][7] ITU World Radiocommunication Conference 2023 (WRC-23), which was held in Dubai (United Arab Emirates) from 20 November to 15 December 2023 formally recognized the Resolution 4 of the 27th CGPM (2022) which decides that the maximum value for the difference (UT1-UTC) will be increased in, or before, 2035.
Since leap seconds are known only 6 months in advance, time intervals for UTC dates further in the future cannot be computed.
[citation needed] Since there is no standard governing this sequence, the timestamp of values sampled at exactly the same time can vary by one second.
[88] Due to a software error, the UTC time broadcast by the NavStar GPS system was incorrect by about 13 microseconds on 25–26 January 2016.
[89][90] The most obvious workaround is to use the TAI scale for all operational purposes and convert to UTC for human-readable text.
The Society of Motion Picture and Television Engineers (SMPTE) video/audio industry standards body selected TAI for deriving timestamps of media.
[63] Amazon followed a similar, but slightly different, pattern for the introduction of the 30 June 2015, leap second,[93] leading to another case of the proliferation of timescales.
