The day a minute had 61 seconds: why leap seconds stop in 2027

BOTH Chochim editorial team · November 10, 2026 · Updated November 12, 2026

Written as of November 10, 2026. The decision to stop leap seconds follows the resolutions of the 27th (2022) and 28th (2026) General Conference on Weights and Measures (CGPM) published by the BIPM; leap second announcements follow IERS Bulletin C 72; the basics follow NIST. The official text of the resolutions is in French; this article uses the BIPM English version. The resolution does not say how clocks will be reconciled once the gap nears one hour, so we do not speculate. The count of 27 leap seconds and the Korea time conversions are calculated from official figures.

On New Year's morning 2017, just before 9 a.m., clocks in Korea did something odd. After 8:59:59 a.m., they did not go to 9:00:00. Instead there was an extra second, 8:59:60, a time that normally does not exist. That minute had 61 seconds.

That extra second was a leap second. In October 2026, the General Conference on Weights and Measures, which sets the world's measurement standards, decided that no leap seconds will be added from May 20, 2027. Why end a rule used for more than 50 years? It starts with the fact that the Earth is not as good a clock as we assume.

The Earth is not a perfect clock

For most of history we measured time by the sun. A day, an hour and a second were slices of one turn of the Earth. Then atomic clocks arrived in the 20th century and exposed a problem: atomic clocks tick with remarkable regularity, and the Earth does not.

NIST explains that Earth's rotation speeds up and slows down, with a long-term trend of slowing. On top of that, the atomic second was slightly shorter than the "solar" second from the start. Follow atomic clocks alone, and clock noon slowly drifts away from sky noon.

The compromise is Coordinated Universal Time (UTC), the world's reference time. It follows atomic clocks, but whenever the gap with Earth-rotation time (UT1) threatens to exceed 0.9 seconds, one second is inserted. The system began in 1972 with a one-time 10-second correction, and since then single seconds have been added.

Leap seconds go in at the end of June or December, just before midnight UTC: 23:59:59, then 23:59:60, then 00:00:00. Korea is nine hours ahead of UTC, so for Koreans it falls at 8:59:60 a.m. on July 1 or January 1. That 61-second minute on New Year's morning 2017 was exactly this.

27 leap seconds, and the last one in 2016

How many leap seconds have there been? Bulletin C 72, issued on July 6, 2026 by the IERS Earth Orientation Center at the Paris Observatory, gives the difference between atomic time (TAI) and UTC as 37 seconds since January 1, 2017. Subtract the original 10-second correction of 1972 and you get 27 leap seconds. NIST says they came about every year and a half on average.

But there has not been one since December 31, 2016. The IERS announces each decision six months ahead, and Bulletin C 72 states that no leap second will be introduced at the end of December 2026. The next opportunity would be the end of June 2027, after the May 20 cutoff. If the resolution is followed, the second added at the end of 2016 is likely to be the last.

It's only one second. Why go to such lengths to get rid of it? This is where computers come in.

The day one second tripped up computers

A 61-second minute means little to people, but a lot to software. Many programs assume a minute has 60 seconds and that time only moves forward. A time like 23:59:60 is simply not expected.

That is what happened at the end of 2016. Cloudflare, an internet infrastructure company, wrote that when the leap second arrived at midnight UTC, a number in its DNS software that should never have been negative went negative. DNS works like the internet's phone book, turning names into server addresses, and some lookups for websites failed. In Korea it was around 9 a.m. on New Year's Day.

The 2022 CGPM resolution put the same concern on record: leap seconds create discontinuities that risk serious malfunctions in critical digital infrastructure, including satellite navigation (GNSS), telecommunications and energy transmission. Worse, operators had developed different ways of inserting the leap second, with no agreed standard.

Google is the best-known example. Since 2008 it has not added the leap second all at once but spread it out, a technique called leap smear. Its recommended method is a 24-hour linear smear from noon to noon UTC, lengthening each second by about 11.6 parts per million. At the moment of the leap, Google's clocks are therefore close to half a second away from official UTC. Google advises against mixing smeared and non-smeared time servers, because the results during a leap second may be unpredictable.

Chochim checkpoint In the leap second era, the same instant could read almost half a second differently depending on which time server you used. For an on-the-dot click, half a second is a lot. Once leap seconds stop in May 2027, that source of disagreement mostly goes away. But the gap between your phone and a ticketing server's clock can appear for other reasons, so checking the server time just before the opening is still the safest habit.

What changes on May 20, 2027

The decision came in two steps. In November 2022, the 27th CGPM decided that the maximum allowed difference between UT1 and UTC would be increased in or before 2035, so that UTC could run continuously for at least a century. The exact number was left to the next meeting.

That meeting, the 28th CGPM, took place on October 13–15, 2026 in Versailles, France. Its Resolution 5 decided two things: continuous UTC becomes effective on May 20, 2027, and the maximum difference between Earth time and UTC will be 3,600 seconds, or one hour. The BIPM says this ensures the continuity of UTC for several centuries. The limit goes from 0.9 seconds to 4,000 times wider.

And here is the twist. Over the long run the Earth is slowing, but lately it has been spinning slightly faster. According to the resolution, a March 2025 workshop estimated that the probability of a negative leap second, removing a second, would rise quickly and reach 30% by 2035. A negative leap second has never been applied, and no one could promise that systems would cope with skipping 23:59:59 entirely. The resolution calls it a high risk for critical infrastructure, part of why the decision moved forward.

So what happens when sun and clock drift close to an hour apart? That is centuries away, and this resolution does not say how it will be handled. Little changes right away, either. The resolution notes that the International Telecommunication Union (ITU) is studying updates to how radio time signals transmit the UT1-UTC difference, and that a transition period runs until 2035 (no later than 2040) where existing equipment cannot be replaced sooner.

Frequently asked questions

When are leap seconds being abolished?

Under Resolution 5 of the 28th CGPM (October 2026), continuous UTC with no leap seconds begins on May 20, 2027. Since the IERS has announced no leap second for the end of December 2026, the December 31, 2016 leap second is likely to be the last if the resolution is followed.

How many leap seconds have there been?

Twenty-seven, calculated from the UTC-TAI value of -37 seconds in IERS Bulletin C 72 minus the initial 10-second correction of 1972. All were positive; a negative leap second has never been applied.

Will clocks drift away from the sun without leap seconds?

Yes, very slowly. The allowed gap has been widened to one hour (3,600 seconds), so UTC can run without interruption for centuries. It is not something you will notice in daily life.

Does this affect checking server time?

It makes things simpler, since servers no longer handle leap seconds in different ways. Device clocks drifting by a few seconds is a separate problem, so it is still worth syncing to standard time before an on-the-dot click.

In short, the leap second was a 50-year promise to keep atomic clocks within 0.9 seconds of the Earth. It became too expensive a second for computers, so clocks will now follow the Earth a little more loosely. The same conference also adopted a resolution on the future definition of the second; how the second itself is defined is the story of How atomic clocks define the second.

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