No leap second will be introduced at the end of December 2026
datacenter.iers.org
datacenter.iers.org
"Because the Earth's rotational speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and not precisely predictable."
Earths rotation has been unusually fast lately. So there is not enough drift to warrant a leap second.
Pandemic -> more people working from home -> less people in tall office buildings -> faster rotation (like a skater pulling in their arms).
Probably not remotely true but it would be funny.
My best guess would be it's somehow related to water distribution? More water going into the atmosphere? Glaciers growing (unlikely)? Did multiple huge water reservoirs go into service and get filled up over the summer months?
Fun fact: leap seconds will stop being a thing soonish. I think they phase out in 2035, with a delay because Russia needed time to update glonass satellites.
(Note: on mobile, this is from memory, details need checking ;))
A frightening fact, the 2011 magnitude 9.0 Tohoku Earthquake shifted the position of the Earth's figure axis about 17 centimeters, making days about 1.8 microseconds shorter.
Everybody agreed that "Leap seconds" are a sufficiently bad idea that they should be replaced by 2035. Nobody has agreed how to fix it, and "Just turn them off" isn't technically legal. However, "What if there were Leap hours instead?" is technically legal and of course those hours would happen in the very distant future (likely after our civilisation is gone) so it's functionally identical to "Just turn them off" but without legal problems.
Now, I'm English, and England loves this sort of hack. You may have heard that controversial UK politician Nigel Farage "resigned" as a Westminster MP recently and that's not technically true because you can't resign, historically people hated that job and so you can't resign and we never changed that, but what you can do, and everybody does, is get assigned an "Office of profit" in which legally the King is paying you, an MP can't work for the King so you can't be an MP any more. The "Offices of profit" in question aren't real jobs† and don't pay real money, like this "Leap Hour" they'd be a legal fiction. So everybody says you "resigned" but in fact you legally can't do that...
† I mean, historically they were real jobs that made sense which is why the King paid somebody to do them, but England is very, very old so they haven't made sense for centuries and serve only as a legal fiction today.
There's a long tradition in the UK of having electoral candidates who don't expect to win but run because it's free publicity in a high profile race. "Count Binface" is a comedian who dresses up as a space alien whose outfit resembles well, having a Bin for a face. The serious political parties told Nigel to fuck off, if he wants to step down and then immediately contest the same seat they wouldn't run against him in this farce, but Binface isn't a serious politician so he is running in that by-election.
Nigel wanted to be able to do this whole thing about how the establishment is rotten and he (Wealthy public schoolboy who keeps lying to people and doesn't bother going to Parliament even though he was elected to do so) is a true man of the people and can put things right. It got him this far in life. But with the other candidate on your ballot being a space alien it's obvious which of these options is really "the establishment" and it's not the guy whose policies include "Building at least one house†" and who says he comes from a different planet...
† British political parties often insist they will build lots of housing because that's popular with voters. But, in practice they don't tend to really deliver because the various groups lobby not to actually build. So "at least one house" is a joke about this phenomenon, while conveniently also being technically possible, Binface could just build a house, that's a thing you can do.
The liberal elite of plumbers and comedians are
Then everything would theoretically be correct. The logical next thing to do would be to move all the time zones as well. But time zones already don't coincide with the lines of longitude in practice; they tend to follow country or internal boundaries somewhat close to the lines of longitude (but sometimes multiple hours away!). After a few thousand leap seconds, maybe one or two countries would feel it was helpful to readjust a time zone boundary to better align with solar time, but in practice this would never be the overriding reason for that decision.
You say people's GPS systems would all suddenly be wrong because they depend on locations in latitude and longitude? I don't think this is a problem either: in practice longitude and latitude are given not relative to the steel bar in Greenwich, but to per-continent geodetic datum points. This already prevents continental drift from affecting your coordinates, though a big earthquake can still mess things up.
For those unaware, the major parties have declined to participate in the by-election triggered by Farage's resignation seeing the whole thing as a farce. As a result Farage will likely face only Count Binface, a space warrior from Sigma Six. He'd get my vote purely on the basis that he's promised to bring back Ceefax, and build at least one affordable house.
It's more like an atheist refusing to swear an oath before God in a courtroom: even if you agree that the law says you must do so, you might still not want to give God that recognition. But worse, because God might also be the defendant and the judge in this case, and you have to swear not only that He might witness your testimony but also that you pledge allegiance to Him, so swearing that oath really impairs your ability to participate in a fair trial.
So for them rather than for atheists England made it possible to Affirm that you're not lying. This will work in Parliament, and it's pretty routine these days that a new member is like nope, no swearing for me, I can promise I'm not lying but I never swear or I won't swear to God.
However, Parliament does require allegiance to the King because this is a constitutional monarchy, if they wanted to be a Republic they'd get rid of the King as a group, that's not up to you as an individual member. Not much notice is taken of how much you seem to mean it about allegiance to the King, because after all plenty of members are known to hold Republican sentiments, but you are required to say the words unlike the stuff about swearing which is optional.
On the other hand, Binface has not, as I understand it, ever said he would not serve if elected. He's made it clear that he's not from Clacton (or Makerfield) -- because he's a space alien -- but I believe he said if he won he would move there so that's fair enough if the constituents want him. They previously elected Nigel, and he's rarely in either parliament or Clacton so Binface can't be worse than that.
- https://theshovel.com.au/2026/07/09/joke-candidate-to-face-c...
Actually, I'd take issue with describing Sinn Féin as a protest vote at all. They've historically been the only choice that even claim to represent constituents in many areas. And they do seem to do much of the work of an MP (writing letters on behalf of constituents, lobbying government agencies...) they just don't vote or debate.
The key aspect is that they consider the English government to be a foreign government and so they avoid involving it in the work that they do in Ireland for their Irish constituency. Statements about the illegitimacy of their government historically come from conservative English sources. But the fact is those SF debates in their "protest government" formed the foundation of the modern Irish state. They are a protest vote in the same way that the US Constitutional Convention was.
By the same token, they consider legislating on affairs that pertain to the English, Welsh, and Scots to be none of their business. To take up seats in a foreign parliament would be to meddle in the affairs of a foreign, sovereign nation. And that would be hypocrisy!
So for example Órfhlaith Begley was elected MP for West Tyrone. Her voters will have known she's not going to Westminster, and she didn't - but she's not in some parallel institution instead, AFAIK there isn't one. Nicola Brogan represents West Tyrone as an MLA in Stormont, because Sinn Féin does recognise the Assembly and you can't say well but there's a body in Dublin. Dublin doesn't control West Tyrone so what would she even do there?
She just doesn't go into the room which is called the House of Commons and try to speak or vote there, because the armed guards at the door won't let in anyone who doesn't swear allegiance to the King. If swearing allegiance to the King was a requirement to use the email system, then she wouldn't do that either.
I think it's parliament.uk rather than parliament.co.uk by the way.
So the core issue, from the Republican perspective, is that the people of West Tyrone are denied representation at the national level in Ireland by a foreign government. Once that representation is achieved the SF representatives will participate in it. (again, not trying to address the merits; just clarify the logic)
I did not expect this to be a real person. Is he with the Standing At The Back Dressed Stupidly And Looking Stupid party?
That would be the Monster Raving Loony party who will apparently also be standing in this by-election. Count Binface has ruled out a pact with them.
I thought this was an opportunity for them to be tactical, but no.
(this is a joke)
His honour Count Binface is from Sigma IX not Sigma 6! To lump him in with those scurrilous, pro-littering hoodlums is the kind of anti-Recyclon smear I would associate with Sigma X’s online forums, not this place!
There were arguments that the government should refuse Farage's appointment because he's doing it to stop the clock on investigation into his various financial dealings. While against constitutional law to decline such, it was discussed in similar situations in the past - fairly recently in fact for the same reason -- Henry Cadogan in 1842
On the subject of headwear in parliament, I quote the member for Hereford who yesterday in parliament said:
> How very different from the forthcoming by-election in Clacton, which appears to be a choice between a novelty comedy act with no real policies, and Count Binface. It is a long time since we had a count in the House of Commons, and when the time comes—as it surely will—we will have to leave to you, Mr Speaker, the delicate question of whether and how to suspend the rules on headgear in the Chamber for the new Member.
Which implies that the laws around headgear are at the behest of the Speaker.
There is precedent in electoral history for election of people dressed as a figure -- H'Angus the Monkey (a football mascott, not an actual monkey) was elected Mayor. However on the ballot paper his entry was
STUART DRUMMOND Independent
Where as Binface's is
Count BINFACE Count Binface Party
Given Farage received a mere 45% in 2024, and a unity candidate beat an incumbant mp who previosuly had 55% and was mired in a similar scandal back in 1997, it's not impossible.
It's like you buy a cat to help with your rodent problem, figuring the cat will eat mice and isn't much trouble to look after, but after purchasing a cat you find that your problem was actually rats, your cat is terrified of these large dangerous creatures and sometimes gets bitten by them necessitating expensive vet bills and now you need to pay a lot of attention to the poor animal and also now need to buy cat food.
I mean we deal with daylight saving time all the time and I know it's not the same because the leap second affects UTC, not just local time zone, it's just that you are either dealing with monotonically increasing time like epoch, or you are dealing with "human" time and I found no distinction in the latter.
Is it "just" that leap seconds or delay seconds caused problems in epoch to utc conversion? Note the just in quotes, but did I just answer my own question? :)
Those things that really do rely on actual "elapsed time" rather than the difference between two recorded "book times".
Does this happen? Yes, a few times in my career in geophysical exploration - it's why multiple bits of gear are synced to a reference "real clock" which gets logged against the raw GPS epoch time (real time since Sunday last week(?)) and processed "UTC" time (some variation of it).
There's a lot of fiddly pedantic stuff that goes with scientific data recording, timekeeping is but one domain of possible issues.
And I wish we didn't every year!
We also don't deal with DST very well either. You won't believe the amount of programs written that treat local time as monotonic. It causes all kinds of problems and most people roll their eyes when someone who knows pipes up about local time and time zones etc.
If the world weren't entirely reliant on software to the extent it is today (like when leap seconds were introduced in the 70s), it wouldn't matter as much.
The treaty says everybody agrees that this new standard will try to track "solar time" which felt intuitively reasonable. They want something equivalent to the old GMT which was really based on solar time, except more modern. At first the idea was, well, we just work out how fast this damp rock spins more precisely and we can use that to ensure everything works forever.
More precise measurements of the damp rock showed that, annoyingly, Mother Nature did not provide the spinning rock as a precise clock, it spins slower and faster according to a huge number of variables and so the best we can do is measure the spinning against an actual clock. So, "Leap seconds" were born to meet that legal requirement to have UTC match the solar time.
The "leap hour" would likewise fulfil this requirement, just in a deliberately useless way because we actually do not care about precisely tracking solar time. If we did, almost every human in the world would be perpetually annoyed because of course our present system of "time zones" means on average we're at least 30 minutes wrong!
No. Not everybody. I prefer accurate time, and all the complaints I've heard hold little water.
My servers need to timesync forwards and back all the time, eg timedrift. They need to jump to new times, or slowly drift, depending.
VMs can be hypervisor starved, or need to move to a new host.
Servers also need to handle missing time. Any daemon or program which cannot handle this is buggy, broken, and needs to deal.
Leap seconds are just part of all of this, and present no new issues compared to normal time change. I question the capabilities of any engineer who singles out time second as difficult to deal with, time is constantly changing on servers. Constantly.
So back to the start, no... everybody doesn't agree. Google isn't "everybody".
It's infrequent enough that most systems won't bother implementing it, but a big enough time difference that it absolutely must be handled correctly at the right instant.
You'd be setting yourself up for a millennium bug-sized panic every century. And as soon as that the generation that experiences one retires, their successors will start saying "there's not going to be a leap minute for the next couple of decades, and there's no chance our code will still be running then...", and the cycle will repeat. Again and again and again.
The leap hour proposal is better because if it really is still relevant in however-many-thousand years, we can do a long-term plan to handle it which includes giving a century of notice.
The 999 year lease to essentially make land practically freehold is one.
https://en.wikipedia.org/wiki/999-year_lease
It's caused significant controversy in my (former colony) country where all other long-term leases are 99 years. The landowners are insisting that their ancestors were cheated and they want their land back.
GLONASS maybe? or really glasnost era satellites?
Why is this? As leap seconds don’t occur on a regular frequency, I assume they are not hardcoded on the software or hardware on board, but the control centre uploads them on the satellites enough in advance once they have been scheduled. So why can’t the control centre just stop sending those updates?
This type of assumption that was made early in a massive software and hardware project that's now been ossified for ~50 years is going to be hard to change.
What are the satellites doing with the models? They're not deciding leap seconds on their own, I hope. So I don't see why the leap second decision would be locked to low accuracy.
Also I would expect doubling the precision to give you a 3-4x slowdown on the math or adding orders to have less effect, and the amount of available computation spent on those models to be like a tenth of a percent at most, so the extra cycles wouldn't be an issue. What am I missing here?
To me it seems like unpredictability is the only real issue.
recalling that the CGPM at its 26th meeting (2018)
- stated that UTC is the only recommended time scale for international reference and the basis of civil time in most countries,
- recommended all relevant unions and organizations to work together to develop a common understanding on the realization and dissemination of reference time scales with a view to considering the present limitation on the maximum magnitude of UT1 - UTC to meet the needs of the current and future user communities,
Arguably, the only real-world impact this drift has on normal people is GPS, but GPS already transmits an offset from its own clock so that receivers can correct for that. The GPS clock is different both from UTC and TIA.
I don't think we know this do we? we haven't been measuring accurately enough for long enough to be confident that it does in fact cancel out. In fact for the period of time where we've been applying leap seconds they were happening with significant frequency and always in the same direction. It's only been very recently that there's been the realistic suggestion it might drift in the other direction.
If anything people have been suggesting that if we do get rid of leap seconds that we can just wait until the offset is enormous instead (say an hour) because it would take a very long time to happen. But even still, that does actually affect everyday people (because people would surely notice when solar noon is an hour later/earlier). Although pragmatically the obvious solution there is to change timezone instead.
Meanwhile....
International timekeepers to vote on changing the leap second to a leap hour
https://www.scientificamerican.com/article/international-tim... (https://archive.ph/GnQUj https://news.ycombinator.com/item?id=48842329)
In practice it will never affect anyone because it's a legal fiction, but even if you pretend to believe we would actually introduce this "leap hour" it would be in the distant future long after we're all dead and if there are still humans who have any idea the year 2026 happened they're not sure which of Donald Trump, Taylor Swift, Tony Stark and John McClane were real people.
Edited to add:
This is such a ridiculously long time frame that they might not be sure whether we were worried about climate change, for them that's either a disaster they survived (and maybe most didn't) or it's a weird blip in their historical charts which they struggle to explain. Did our civilisation do something very, very stupid? There is a flammable gas deep underground, did we set fire to it because we were crazy? Why the hell would we have done that? There are signs we deliberately set fire to the coal which is a toxic rock also found underground? That would explain the global climate going nuts. Maybe it was a ritual or something. Ancient people are mad.
Even if one day humans have to account for relativity in their commute, their woes will pale in comparison with those of the poor soul who has to add support for it in a C library that only understands (now, 128-bit) Unix timestamps.
The Julian period is a chronological interval of 7980 years, derived from three multi-year cycles: the indiction, solar, and lunar cycles. The last year that was simultaneously the beginning of all three cycles was 4713 BC (−4712), so that is year 1 of the current Julian period, making AD 2026 year 6739 of that Period.
> Even if one day humans have to account for relativity in their commute
You don't think there aren't already application domains that have to account for relativity differences between reference frames?
>You don't think there aren't already application domains that have to account for relativity differences between reference frames?
Of course, GPS for one. My point is about the legacy of it all. Long time after those satellites are down, some future astronomer will be translating timestamps between GPS time and UTC, entirely aware of leap seconds and atomic time and whatnot, just to make sense of 21st century observations.
Save for those that care about missing days and months.
As long as cross country events prior to ~1756 aren't being discussed, things get messy and non uniform fast.
Also, there are Gregorian adjacent calendar variants with a Year 0
> some future astronomer will be translating timestamps between GPS time and UTC, entirely aware of leap seconds and atomic time and whatnot, just to make sense of 21st century observations.
A future where the spin of the earth still isn't a uniform metronome - a future with the same issue that exists today (and last century).
> their woes will pale in comparison with those of the poor soul who has to add support
Much of this support has _already_ been added - SKA data networks, for example, have to account for timing issue caused by receivers on one side of planet turning toward a source Vs those on the other side turning away - and reconcile that with past data from the other side of the orbit when the planet was moving toward Vs now when it moves away.
Nothing I do requires this level of precision, but certainly there are things that do.
(And depending on the leap smearing implementation, it also smears CLOCK_TAI, jumps it to opposite polarity at the actual time of leap second, and then smears it again. The leap smearing people really made a mess of this.)
From a correctness perspective, the only good choice is to go all-in on TAI.
[Ed./P.S.:] "just ignore leap seconds" - that's going all-in on TAI. At this point it's probably easier to redefine UNIX timestamps as TAI based after 2035 ("abolishing leap seconds"), and introduce a new CLOCK_SOLAR_EARTH that accumulates leap seconds and can be used if/where necessary. The main issue is to create a proper delineation between the two clocks, which we just don't have at this point. Way too many systems where it's just not clear what they use.
And note that leap seconds are earth specific too. You'll have entirely different requirements on e.g. Mars.
I don't understand what you mean. Doesn't smearing still mean the clock only goes forward? It's still steadily incrementing. The only difference is that a second is slightly longer/shorter than you expect, but you already have to account for that if you're doing the kind of physics experiment where it would matter.
I'd figured that UNIX time just counts actual seconds and that leap seconds and similar calendar shenanigans would be a problem of mapping epoch to the correct date, so that if normally epoch X maps to date D then both epoch X and X+1 map to date D.
Am i to understand that leap seconds "stretch" a epoch unit ? so that some epoch second "lasted longer".
Yes, 1.0000116s (or 0.9999884s), and to be clear it was never intended like that. It got started after a bunch of bugs in leap second handling caused issues & some people thought it would be better to just stretch/compress time for a period around leap seconds, for docs refer to e.g.: https://developers.google.com/time/smear
Personally speaking - it's a great way of just pushing the problem around, further complicating an already complex situation. 11.6µs is very measurable on a modern system.
> This would make converting from unix time to UTC need extra arbitrary handling for display, at least.
Except you need to know that leap smearing has been applied… which of course noone records. Most systems can't even signal it, much less store. If you need that level of precision, you better make sure none of your systems uses it or you're just screwed.
> 11.6µs is very measurable on a modern system.
11.6µs is very measurable.
Clock skew of 11.6 PPM is much more subtle.
It doesn't. UNIX time is 1:1 with what your clock reads, you can convert between 2026-07-10 00:12:25 UTC and 1783642345 by simple math (note how the last digit is the same; but the leap second offset is 37s, that wouldn't align. The TAI timestamp for the same moment in time is 1783642382.)
> and that leap seconds and similar calendar shenanigans would be a problem of mapping epoch to the correct date, so that if normally epoch X maps to date D then both epoch X and X+1 map to date D.
It's the other way around, if you need to get the exact seconds between two UNIX timestamps (or calendar date/time), you need to check if there were any leap second changes between the two.
Counting actual seconds is TAI [https://en.wikipedia.org/wiki/International_Atomic_Time], that one is really just "number of Caesium hyperfine transitions since X", and now the mapping to a calendar date/time for display is a bit involved.
So the times before and after leap second get converted the the same date regardless of whether you know about the leap second or not. Edge case is that once second occuring twice. You might have seen that 23:59:60 seconds timestamp somewhere.
Unix time assumes a fixed number of seconds per day. 86400. If a leap second is inserted, either end of June or end of December, the day is 86401 seconds long.
There are different implementations for how your Unix time will behave 24 hours before to 24 hours after the leap second. A timestamp might just repeat during the leap second or the system changes the length of a second in a time range around the leap second introduction to make up the difference. This is called smearing.
A simple example is the elapsed time between these two timestamps:
2016-12-31 23:59:50
2017-01-01 00:00:10
2016-12-31 23:59:60
But it is just a basic system clock. Being the wrong speed by 15 parts per million shouldn't throw off your data collection. Lots of clocks are more inaccurate by accident.
- clarification: "fully ignorant" from a human perspective, using dates and times. UNIX time lines up with those.
New leap second will get to your system through NTP. Sadly NTP only distributes indicator flag that leap second is going to be introduced, but not the offset itself. But the distributed time itself is already affected by leap second, so NTP client doesn't really need to know.
(In contrast the other time sync mechanisms - GPS and PTP - use time scale unaffected by leap seconds and distribute it as an additional information with UTC offset. And it's left on client to modify received time on its end. Kernel has a parameters in clock_adjtime() for leap seconds.)
So if you have a passive system that has NTP client then it's time will change for new leap second on runtime. Linux treats UTC time as the dominant one, so that's the one saved to RTC device and will survive reboot.
There is CLOCK_TAI that sounds like it should return TAI time, but it is such a second class citizen to the point that nothing on regular Linux desktop and server distros even set the offset and it returns the same time as CLOCK_REALTIME.
There is a file in /etc with list of leap seconds that is part of some package, so you need to update the system to update this file. I don't believe traditional NTP software updates this package dynamically. But not many software uses it. If some init service script parsed and set the kernel UTC offset then your system's CLOCK_TAI would be one second late from rest of the world until update. But it doesn't affect UTC time on Linux in any way I know.
That the notice comes every 6 months is just to meet the letter of the original international treaty.
The reason we've stopped having leap seconds is because the drift between TAI and UT1 slowed down and has actually been very slowly drifting in the other direction.
I hope they have one of those logos with a bunch of weird freemason symbols that freak people out.
The Time Lords are a fictional ancient race of extraterrestrial people in the British science fiction television series Doctor Who. In-universe, they hail from the planet Gallifrey and are stated to have invented time travel technology.
[Edited to avoid assigning a gender to a character who has had different genders]
Also, not only is “The Doctor” not the character’s real name, they have been called “Doctor Who” multiple times, including in credits, so you pedantry isn’t even right.
https://screenrant.com/doctor-who-name-debate-settle-explain...
(Depends on the era of course, sometimes there are other timelords, sometimes not.)
Even the titles are sci-fi.
[1] https://en.wikipedia.org/wiki/Planetary_Defense_Coordination...
Is it a headache or a non-issue
Some rare systems use monotonic oscillator seconds and ignore the earth rotation second, but if you ever have to translate those to real time, you get an accumulating disaster over time and it’s generally regarded as not a good idea.
I believe in the US this error correction has been discontinued in the East and in Texas, but is still done in the West for some kind of non-clock "inadvertent interchange" reasons I don't understand.
[Spoiler]
They are good reasons. Conspiracy theorists are able to persuade almost everybody that the reasons were bullshit, an excuse to seize power or something, and so the few who still insist this was necessary and mustn't stop are executed. Almost immediately after those executions, Mother Nature proves them right. So that leaves everybody: Guilty of having murdered their saviours and with no choice but to carry on with the very plan they had insisted was bogus...
There is a Chinese movie but I'm kinda surprised no Hollywood studio got themselves a rights deal and made a US-friendly movie where the Sun conveniently blows up slightly earlier and our heroes are vindicated and everybody agrees they were right all along.
In the real world their scheme is doomed as it would strip the atmosphere off of the Earth, but since they were planning to leave the solar system the atmosphere was going to freeze to a solid anyway so maybe it didn't really matter. To be honest I thought the entire scheme was an extremely elaborate cover for the fact the lead ship was the actual ark and everyone else was just plain doomed. This would have mirrored one of the themes in the 3 Body Problem where interstellar space travel is strictly forbidden until everybody can participate at once. The "launch the Earth as a spaceship" concept was so poorly thought out I thought it had to be a fraud in the story, and our somewhat dense protagonists just didn't catch on.
(oh and it wouldn't be strong enough to affect the Earth's rotation)
It would have to be rockets. My napkin says 2B starship boosters and 250K tons of propellant per second should give us around 1 second per year.
>
> from 2017 January 1, 0h UTC, until further notice : UTC-TAI = -37s
This means the atomic clock is behind the solar clock by 37 seconds? I also don’t understand the reference to 2017.
My guess is that is when they last changed the offset, so the -37s has been in effect since then.
It did not happen yet. No negative leap second that should have been inserted was skipped.
See this chart here:
https://en.wikipedia.org/wiki/DUT1#/media/File:Leapsecond.ut...
You can see the the positive leap seconds as jumps upward once the graph reaches around -0.5 s.
We were never close to reaching +0.5 seconds with a positive trend and we are still relatively far away from that.
If anything, it's the other way around.
A UTC day is defined as exactly 86400 SI seconds. But an actual mean solar day is a few milliseconds longer (although the difference is not constant due to irregularities in the Earth's rotation--but the average difference is expected to slowly increase over time). SI seconds are counted by atomic clocks, so UTC advances its day by one every 86400 atomic clock seconds.
But a solar clock that advances its day by one every time the mean sun reaches noon (it has to be the mean sun because the rate at which the actual sun moves across the sky varies over the course of a year, we need to look at the average) will advance its day a few milliseconds later than UTC does. Or, to put it another way, each time period that the solar clock says is exactly 86400 seconds, is a few milliseconds longer according to the atomic clock.
As this happens day after day, the difference accumulates, and when it gets close to being a full second, a leap second gets inserted into UTC, so that one of its days is 86401 seconds long instead of 86400. The reason for this is that UTC is not just counting atomic clock time; it also has to stay in sync with where the sun is in the sky since so many human activities are tied to that. And we humans have defined "in sync with the sun" to be "within a second of the average sun". In other words, we want UTC noon to be within a second of mean solar noon on the prime meridian.
So the 37 seconds is how far mean solar noon would be behind UTC noon, if we didn't use leap seconds--at UTC noon, the mean sun would be 37 seconds short of actually crossing the prime meridian in the sky.
Why?
If I travel 1 mile east or west of the prime meridian, my solar noon now comes 2-3 seconds earlier/later. It's nearly impossible to have your local time match your local solar noon. For most of the population, solar noon is, on average, 30 minutes off of 12:00 noon.
Plus, solar noon varies from day to day by 10-20 seconds. Check the charts out. https://www.timeanddate.com/sun/usa/new-york
Um, because it's the prime meridian and that's how UTC is defined?
> It's nearly impossible to have your local time match your local solar noon.
Which is why I specified on the prime meridian, which is the particular local meridian that UTC is defined as corresponding to.
> solar noon varies from day to day by 10-20 seconds.
Which is why I was careful to specify mean solar noon.
I'm not quite sure what your issue is. Yes, we have time zones tied to specific meridians, and the actual sun's speed in the sky varies (which I mentioned in my post, so I'm not sure why you seem to think I'm unaware of it) so in most places local time by the clock doesn't match local time by the sun. Yes, a leap second adjustment to UTC is quite a bit smaller, taken in isolation, than the annual variation in actual solar time vs. mean solar time.
But over time, if we didn't have leap seconds, the difference would accumulate. The accumulated difference now between UTC and TAI is 37 seconds--which is almost twice the maximum variation in actual solar noon from mean solar noon that you refer to. We humans have collectively decided that we don't want that, and that it's better to do the adjustments a little at a time rather than in bigger lumps.
No, the 10-15 seconds I mentioned is the daily variation in solar noon.
From the link I posted, in NYC, solar noon on 2026-01-01 is at 11:59am. On 2026-01-31, solar noon is at 12:09pm. In one month, it has drifted 10 minutes. That's much greater than the 37 leap seconds we have added in 60 years.
"We humans have collectively decided that we don't want that, and that it's better to do the adjustments a little at a time rather than in bigger lumps."
Yet we just reversed that decision. No more leap seconds after 2035. After trying it, we decided it was terrible.
Yes, but averaged over an entire year, it still comes out to zero. The difference between mean solar and atomic time does not. It accumulates over the years.
> we just reversed that decision
We paused it for 100 years after 2035. That doesn't change the physical fact that the Earth's rotation will continue to slow over the long term. We might eventually decide to just not care about that when it comes to civil timekeeping, but that's not what the decision you're referring to did. It just said we can afford to let the difference between UTC and TAI accumulate from 2035 to 2135 (by which time it is predicted to be about a minute) while we figure out what we want to do over the longer term.
That's an explanation of how it is, not why we should care to preserve it.
The definitions of hours minutes and seconds have changed before, and in recent history.
> Which is why I was careful to specify mean solar noon.
And "mean solar noon" is meaningless to people's lives. Even in the areas where time zones do follow meridians and not country borders that are many minutes off.
In terms of what physical process we use to set the standard, yes. But those very changes were made to try to preserve the same time periods that were important to humans. In other words, to not change what hours, minutes, and seconds mean intuitively to us humans as we go about our daily lives.
Would you also say that getting rid of leap seconds and allowing UTC to gradually drift away from the sun is making the technical side easier at the expense of relatability?
And that's exactly why the SI second has the length that it has--to be as close as possible in terms of how atomic clocks work to 1/86400 of an Earth mean solar day at a chosen epoch. (Note that the actual definition before the atomic clock one was adopted was in terms of the Earth's tropical year at that epoch--but the fraction of the tropical year that was chosen was to line up the second with 1/86400 of an epoch day.) If we didn't care about "relatability", nobody would have gone to all the trouble of trying to determine how many cesium clock oscillations there were in 1/86400 of an epoch day.
Ok so far.
> and you can divide that by 60 twice
I'm not so sure. The concept of 24 hours in a day originates in civil timekeeping, yes, but not minutes and seconds. Those originally came from astronomy, and corresponded to angles, not times, and those angles are constants; they don't change as the Earth's rotation slows down or as its speed in orbit around the Sun changes over the course of a year. I don't think people's intuitive concept of minutes and seconds is that they vary according to the time of year or the tidal effects on the Earth.
> I don't think people's intuitive concept of minutes and seconds is that they vary according to the time of year or the tidal effects on the Earth.
But if you asked people to choose between "seconds vary by an imperceptible amount, less than your clocks naturally drift" and "an hour doesn't have 3600 seconds" I bet most will pick the former.
And it doesn't have to drift day by day, you can average it over a multi-year period.
It's going to be weird once the earth slows down enough that we'd need a leap second every day or two. Once you can't ignore the drift anymore, the system we chose is significantly unintuitive.
> If we didn't care about "relatability", nobody would have gone to all the trouble of trying to determine how many cesium clock oscillations there were in 1/86400 of an epoch day.
I'm not saying we didn't care about it, I'm saying we didn't put it as top priority. We went with a nice clean fixed-length second that will drift away from the Earth over time. And it wasn't/isn't that hard to determine the number of oscillations since "matching" the Earth's unstable speed has a huge margin you can land within.
> Would you also say that getting rid of leap seconds and allowing UTC to gradually drift away from the sun is making the technical side easier at the expense of relatability?
Yes.
I'm in favor of both, to be honest. But it's a tradeoff, and it's a tradeoff that weakens the layman's definition of a second.
They should live in the same abstraction layer that does leap days and daylight savings: the time zones.
The point is that it's weird that we handle a day every 4 years off in a different way to a couple of second being off.
The notable differences are that
1) the leap second happens at the same time globally (23:59:60 UTC), while leap days start at 00:00 local time
2) leap seconds happen at irregular intervals
3) leap seconds are nearly universally implemented wrong, because the ability to show :60 on a second display for for one second at most twice per year is just not worth the implementation complexity
You could argue about 1, but the alternative would lead to much more complicated timezone math (time zones can be an additional one second apart from each other depending on whether the leap second is already applied) for very limited benefit. Number 2 seems unavoidable, and 3 is entirely unintended, just the way things have worked out in real life
We can imagine a system where leap days are split into mean and variance: This would look like a council coming together every thousand years to decide if that year will have a leap day or not, but otherwise we follow the pattern.
We can also imagine a system where leap seconds are split into mean and variance: Many years from now when the Earth is notably slower, there's a guaranteed leap second every odd month, and sometimes there's an extra leap second in June.
My ideal year is 12 months, each 5 weeks long, each week 6 days long. At the summer solstice, 3 intercalary days (bank holidays), at the winter solstice, 2 or 3 intercalary days depending on leap year.
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[0] https://en.wikipedia.org/wiki/International_Earth_Rotation_a...
Most datetime APIs are fundamentally designed and intended for supporting calendar and wall clock operations for business functions. If you need SI seconds for scientific purposes, you really need to use alternative APIs and facilities that provide and guarantee the semantics required all the way down to the hardware level. Likewise, if you want timers, etc, for software facilities like thread sleeping, you use dedicated interfaces like monotonic clocks. If leap seconds are phased out, this won't really change the situation. It was wrong for software to rely on Unix timestamps for, e.g., mutex algorithms before and it'll be wrong if and when leap second clock adjustments are gone.
we were all waiting for the negative leap second to finally happen - but cowards got too afraid