Talk:Leap second

IMO, the lead as it stands at present doesn't meet the standards of WP:LEAD. It doesn't provide a succinct summary of the article and gets bogged down in detail. I have drafted this replacement text. As it is a significant change, I thought I should check first whether anyone disagrees. (Some of the details that I weeded out will need to be relocated, probably at history.

A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (as measured by atomic clocks) and imprecise observed solar time (known as UT1 and which varies due to irregularities and long-term slowdown in the Earth's rotation). The UTC time standard, which is widely used for international timekeeping and as the reference for civil time in most countries, normally uses precise "atomic time" and consequently would run ahead of observed solar time, unless reset occasionally. The UTC specification requires that this difference should not become significant: the leap second facility exists to provide this adjustment.

When it is mandated, 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.^[1] All leap seconds (as of 2019) 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 atomic time 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,^[2] 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.^[3][4]

This practice has proved disruptive in the modern era, particularly in services that depend on rigorous time stamping or time-critical process control. The relevant international standards body has been debating whether or not to continue the practice with an increasing number of nations supporting its abolition.

Any objections? --John Maynard Friedman (talk) 21:57, 1 November 2019 (UTC)

I read the reference at [2] and don't see any mention of climatic events affecting Earth's rotation. Are you sure they have a measurable contribution? 31.168.44.122 (talk) 08:39, 22 December 2024 (UTC)

The debate about the lead finished some time ago but, since you raised the point, here is a recent reference: Climate change is causing days to get longer by slowing down the Earth (Natural History Museum). It is the effect seen when an ice-skater spins on the spot with arms up v. outstretched.--𝕁𝕄𝔽 (talk) 13:01, 22 December 2024 (UTC)

"It's an ill wind" etc. The article needed to be updated with that info, which I have now done. So thank you for raising it. --𝕁𝕄𝔽 (talk) 16:02, 22 December 2024 (UTC)

I suggest removing “normally” from the first paragraph: UTC uses SI seconds, by its definition. I also think the last paragraph could be eliminated from the lead, and moved to the beginning ofthe section on problems with leap seconds. If the lead is too long, even with the last paragraph eliminated, I think keeping just the first paragraph would be good: it clearly and succintly describes leap seconds. John Sauter (talk) 07:19, 2 November 2019 (UTC)

Thanks John Maynard Friedman. This looks like an improvement. I support replacing the current lead.

I think we should keep the last paragraph in your proposal as that topic accounts for more than half the body of the article at present. Maybe that's WP:UNDUE or WP:RECENT but that's where we are and the lead should summarize the article. ~Kvng (talk) 16:40, 2 November 2019 (UTC)

Thank you both. I really had doubts about keeping the second paragraph but thought perhaps I might be going too far. I am encouraged to be bold and chop it. Since moves to eliminate the leap second (and the problems they have caused) take up a good chunk of the article, a mention in the lead seems essential to me too, especially as it is only two lines. But "modern era" is a bit pompous, I'll change it to "the twenty first century" (I would prefer "recent years" but that would just attract a "how recent" tag!)--John Maynard Friedman (talk) 21:52, 2 November 2019 (UTC)

Ladies, Gentlemen,

Please let me tell you that according to IERS/EOC plot, from early June since May 2020, UT1-UTC increases. Under this condition negative leap second at the end of 2023 the decade should be expected. Anyway, today, negative leap second is more probale than positive one. Then I propose some reference in the article.

With regards and friendship, Georges Theodosiou, The Straw Man Georges T. (talk) 14:04, 16 September 2021 (UTC) 14:20, 18 September 2021 (UTC) 13:53, 21 September 2021 (UTC)

Right now, that interpretation (though reasonable) is wp:Original research for as long as it is you who is drawing that inference. We have to wait until a seriously reliable source says so: a scientific journal not a newspaper. My own guess is that there will be very strong political pressure to avoid doing it, because of the chaos it will cause in a wide variety of systems because it will mean that events that are actually a second apart will be recorded as having occurred simultaneously. --John Maynard Friedman (talk) 14:59, 16 September 2021 (UTC)

Mr. John Maynard Friedman, please let me express you many thanks for you commented my proposal. Also, please feel free to delete this section. With regards and friendship, Georges Theodosiou, The Straw Man, Georges T. (talk) 13:21, 17 September 2021 (UTC)

Mr. John Maynard Friedman, please let me tell you that now, after my edits in title and in my first message, it is Mr. John Sauter's Original Research. Just my latest research agrees with his. Georges T. (talk) 12:21, 18 September 2021 (UTC)

I think a more likely date for the negative leap second is around the end of this decade. I also predict that there will be no serious effort to abolish leap seconds, in spite of the chaos that will be caused by poorly-written software, until three or four months before it happens. My evidence is the similar problem with the year 2000: in spite of the fact that everyone saw it coming, some were unprepared, and there were some glitches due to software that couldn't handle it. My hope is that there are some people who remember the year 2000 and will devote some effort between now and then to testing and fixing the important software. John Sauter (talk) 14:19, 17 September 2021 (UTC)

It is not just an issue of software: for seriously time-stamp critical applications, there is no obvious workaround. Well not that I can see anyway but greater minds that mine may find a way. So my guess is that nothing will be done: world-wide consensus will be wait and see if it rights itself over a 25 year horizon and just ignore these short term perturbations. In almost all contexts, atomic time is preferable anyway: until the deviation exceeds say five minutes, hardly anyone will notice the difference from solar time. Maybe there will be a "UTx" for solar time but it won't be adopted as civil time anywhere. --John Maynard Friedman (talk) 15:06, 17 September 2021 (UTC)

Serious time-stamp critical applications should be written to work in the presence of positive and negative leap seconds. In today's world, everything is software, and the software can be written correctly. It isn't actually hard to do, it just takes the will to do it. John Sauter (talk) 02:49, 18 September 2021 (UTC)

Mr. John Sauter, please let me tell you that since negative leap second has never been introduced, it is terra incognita for the engineers, as in programming "undefined behavior" and "unexpected results" should always be expected. That's the problem. Georges T. (talk) 12:21, 18 September 2021 (UTC) 13:53, 21 September 2021 (UTC)

According to "Future of Leap Seconds" website, there are potential legal issues that would occur in countries like USA, Great Britain, and Canada, that still regard GMT (i.e. UT1) as legal time. For example, an accident recorded on camera, occurring seconds from midnight, may or may not be subject to insurance payout if insurance expires on that midnight. So I would say there are definitely potential issues, and they are not necessarily related to software only. Morycm (talk) 00:22, 1 March 2024 (UTC)

Mr. John Sauter, please let me express you my many thanks for you commented my proposal. Regarding situation in 2000 I post this plot. It is clear that there was not any increase similar to that since June 2021. By the way, let me suggest you and every interested this IERS/EOC page for retrieving data and plots. My way for predictions under aforementioned condition is that in 2 months (June 17 to August 17) increase was 44 msec that is 22 per month. With regards and friendship, Georges Theodosiou, The Straw Man Georges T. (talk) 14:52, 17 September 2021 (UTC)

The Earth generally speeds up during Northern Hemisphere Summer but slows down in Winter, so projecting using just Summer data will give you too early a date for the negative leap second. My reference to the year 2000 was to software and hardware which assumed that the first two digits of the year would always be 19. John Sauter (talk) 02:43, 18 September 2021 (UTC)

(As in the year 2000 problem.) Guy Harris (talk) 08:53, 18 September 2021 (UTC)

Mr. John Sauter, please let me agree on that "The Earth generally speeds up during Northern Hemisphere Summer but slows down in Winter, so projecting using just Summer data will give you too early a date for the negative leap second". Whoever as it seen in this plot, UT1-UTC bottomed on May 2020. Since then it rises in mean terms. Its rising ratio is about 70 msec per year. Under this condition you are right on that "a more likely date for the negative leap second is around the end of this decade", my estimation is the end of 2033 2032, indeed if ITU fail solve the problem till then. With regards and friendship, Georges Theodosiou, The Straw Man, Georges T. (talk) 09:28, 18 September 2021 (UTC) 07:51, 27 September 2021 (UTC) 07:43, 8 October 2021 (UTC)

@Georges T.: For future reference, please do not edit your comments on a talk page the way you did above after other users have replied to those comments. Any edits you make to your comments after discussion has started should be accomplished by striking out (using <del>...</del>) the words you want to replace and inserting (using <ins>...</ins>) the words you want to substitute in. See WP:TALK#REVISE for details. - Donald Albury 13:40, 18 September 2021 (UTC)

Mr. Donald Albury, please let me express you my sincere gratitude for your suggestion. With regards and friendship, Georges Theodosiou, The Straw Man, Georges T. (talk) 14:26, 18 September 2021 (UTC)

Ladies, Gentlemen, please let me report you (IERS snob my messages) something strange. Despite their name (International EARTH ROTATION Service), they do not (actually they do as Δω₃, I apologize) publish in their proper page data and plots for EARTH ROTATION speed (omega). Though omega is UT1's independent variable according to Newcomb's formula (omega replaces sidereal time), UT1-UTC depends on two speeds: omega and cesium atom's oscillation. It follows, monitoring UT1-UTC (or UT1-TAI) we do not monitor omega. With regards and friendship, Georges Theodosiou, The Straw Man Georges T. (talk) 07:25, 29 September 2021 (UTC) 09:50, 27 April 2022 (UTC)

The General Conference on Weights and Measures on 18 November 2022 decided to abolish the leap second no later than 2035 (reported by Nature). US law recognizes the CGPM as interpreted by the Secretary of Commerce and Secretary of the Navy as the competent authority to define UTC. But the US is also a member nation of the ITU, and the ITU is responsible for defining how UTC is disseminated. So what steps will we be seeing to actually carry out the CGPM decision?

One step that will be needed will be to alter the way WWV and similar time signals disseminate the difference between UT1 and UTC. Currently this is done by lengthening certain time ticks to indicate the number of deciseconds that should be added to or subtracted from UTC to obtain an estimate of UT1. It is only defined for the range ± 0.9 s. Options include changing the format of the signal, eliminating that feature of the broadcast altogether, or shutting down WWV and WWVB altogether, as was proposed during a recent budget fight. Jc3s5h (talk) 13:33, 19 November 2022 (UTC)

At the risk of drifting into WP:NOTFORUM, I suggest that the message "by 2035 at the latest, could be earlier" is telling the technical committees to produce a plan to make it happen. For WP purposes, we can only wait and see how they resolve it. --𝕁𝕄𝔽 (talk) 13:42, 19 November 2022 (UTC)

Trying to avoid WP:NOTFORUM, if we deem the CGPM decision sufficient to assure that the leap second will be abolished, it will change the way we describe leap seconds and UTC in articles. We would have to regard leap seconds as a method of aligning UTC to UT1, used from 1972 until a date to be determined, rather than as a method that will continue indefinitely. And the political reality is that after 2035 UTC might be an offset version of TAI, or might be a variant of Universal Time such that |UTC - UT1| < n, where the value of n has not been decided upon. Jc3s5h (talk) 13:56, 19 November 2022 (UTC)

Yet another possibility is that it will not be possible to get all the details worked out by 2035, so leap seconds will continue until they are. John Sauter (talk) 15:39, 19 November 2022 (UTC)

It seems that Nature is not convinced that the decision is final either. --𝕁𝕄𝔽 (talk) 11:14, 21 November 2022 (UTC)

In the History section, shortly after "rubber second": The phrase "slower than the rate of atomic time by −150 parts per 10 (raised to the tenth)" is confusing. Is that dash a "minus sign/negative sign"?

If so: Doesn't slower by a negative amount mean faster? If "slower" is kept, I suggest removing the negative signs from the various phrases in this section. David10244 (talk) 05:32, 24 January 2023 (UTC)

I have changed "slowed" to "offset", using words from the source, to make the meaning clearer. John Sauter (talk) 06:25, 24 January 2023 (UTC)

That sounds better, thanks. David10244 (talk) 05:05, 25 January 2023 (UTC)

In the Process section, in the phrase "in 1972 the average length of day was approximately 86400.003 seconds and in 2016 it was approximately 86400.001 seconds, indicating an overall increase in Earth's rotation rate over that time period".

It seems like there's a decrease, not an increase, in the rotation rate, since there's a decrease in the "length of day"... if the length of a (rotational) day is measured in "atomic seconds". But I could be reading this wrong -- not sure. David10244 (talk) 05:44, 24 January 2023 (UTC)

I believe you are reading this wrong. Consider a simpler example: a ball spins at a rate of one rotation every 2 seconds. In order to make the ball spin at a rate of one rotation every second (decreasing the rotation time) you must increase the rotation rate. John Sauter (talk) 06:29, 24 January 2023 (UTC)

Ah, yes, that makes sense. I was backwards on this one. David10244 (talk) 05:06, 25 January 2023 (UTC)

I notice the date format in the article is inconsistent, especially in the references. I found the first edit that introduced dates that contained both the day and the month. It used the dmy format so I have put the {{use dmy dates}} near the top of the article. I will look for inconsistent dates and fix them. Jc3s5h (talk) 16:11, 13 July 2023 (UTC)

I think there's a leap second on June 30th, 2026, pretty sure it's the People there that annonced that it's not happening on December. 2603:7000:C7F0:E5C0:49F6:DF9E:6832:D0D0 (talk) 00:44, 1 October 2025 (UTC)

The official announcements that there will, or will not, be a leap second may be found at the page for IERS Bulletins. Bulletin C is the one for leap seconds. Just click on the link for the latest bulletin and you will see there is no leap second at the end of December 2025, and nothing is said about June 2026. Jc3s5h (talk) 02:19, 1 October 2025 (UTC)

The sentence "The goal is to reduce the frequency of leap seconds by widening the tolerance, rather than continuing to insert them indefinitely." is not supported by a reference and makes no sense to me. Yes, widening the tolerance will likely reduce the frequency of leap seconds, but they will still be inserted indefinitely. If there is no objection I will delete this sentence. John Sauter (talk) 13:24, 22 November 2025 (UTC)

Having heard no objections I have deleted the sentence. John Sauter (talk) 21:19, 6 December 2025 (UTC)