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The end of the leap second: what was decided and what the leap hour means

⏱️ 8 min read

Leap seconds will no longer be added to the world's time. The General Conference on Weights and Measures (CGPM), which met in Versailles from 13 to 15 October 2026, adopted a resolution under which Coordinated Universal Time (UTC) will run continuously from 20 May 2027. The gap between UTC and the time defined by Earth's rotation will be allowed to reach one hour. Until now it was kept below 0.9 seconds.

The last leap second was added on 31 December 2016. The International Earth Rotation and Reference Systems Service (IERS), which announces leap seconds, has already confirmed that there will be none at the end of December 2026. News reports talk about a “leap hour”, but the resolution does not schedule a one-hour correction for any date. Here is what is actually changing, and why.

Last leap second: 31 December 2016. Continuous UTC: from 20 May 2027.
  1. first leap second in UTC
  2. last leap second added
  3. CGPM: the limit will rise by 2035 at the latest
  4. CGPM: resolution on continuous UTC and a 1-hour limit
  5. UTC without leap seconds
  6. original deadline set in 2022
  7. end of the ITU transition for older radio equipment

Spacing between points is not to scale.

What exactly was decided

Resolution 5 of 2026 is titled “On the technical actions needed to ensure the continuity of UTC”. Its operative part has two points:

  • continuous UTC will take effect on 20 May 2027,
  • the maximum allowed difference between UT1 and UTC will be 3,600 seconds, or one hour. According to the resolution, this keeps UTC continuous for several centuries.

20 May is World Metrology Day, which marks the signing of the Metre Convention in 1875. The resolution was adopted at the 28th meeting of the CGPM.

The conference was following through on its 2022 decision. Back then it resolved that the limit would be raised in or before 2035, so that UTC could stay continuous for at least a century. The new resolution brings that date forward by several years. It also asks the International Bureau of Weights and Measures (BIPM) to keep monitoring the difference between UT1 and UTC using IERS data and to report on it regularly to the International Committee for Weights and Measures.

The International Telecommunication Union (ITU) has planned a transition too. Radio time signals broadcast the UT1 minus UTC correction, and the ITU is working on a new way to encode it. Radio systems have until 2035 to start broadcasting the larger difference, and until 2040 at the latest where existing equipment cannot be replaced sooner.

What a leap second is

Two time scales run side by side. International Atomic Time (TAI) is kept by atomic clocks and runs uniformly. UT1 describes Earth's actual rotation, which is not uniform.

UTC, the basis of civil time in most countries, is a compromise between them. It runs at the same rate as TAI but differs from it by a whole number of seconds. Those seconds were added to keep UTC close to UT1. Whenever the difference between UT1 and UTC was predicted to approach 0.9 seconds, a leap second was inserted into UTC.

Every leap second was inserted at the end of June or December. The last minute of such a day had 61 seconds: after 23:59:59 a UTC clock showed 23:59:60. Poland uses UTC plus one hour in winter, so there the last leap second came at 00:59:60 on 1 January 2017.

From 1972 to 2016, 27 leap seconds were added, roughly one every year and a half on average. The longest pause lasted seven years, from the start of 1999 to the end of 2005. The difference between TAI and UTC is 37 seconds today, not 27, because it was already 10 seconds at the start of 1972 (IERS history of TAI minus UTC).

Why leap seconds were needed

The US National Institute of Standards and Technology (NIST) gives two reasons. The first lies in the definition of the second itself. The atomic second was calibrated by comparing cesium clocks with the old Ephemeris Time scale, and the ephemeris second was slightly shorter than the mean solar second. That difference carried over to the atomic second. According to NIST, had it been based on the mean solar second, leap seconds would probably have been needed much less often.

The second reason is Earth's rotation itself. Its rate is not constant: it sometimes rises and sometimes falls, and averaged over long periods it is gradually slowing. As a result, the mean solar second is slowly growing longer relative to the atomic second.

Is Earth speeding up?

In recent years, yes, though not steadily. IERS length-of-day data show this. We look at the period from 1992 onwards, because since then the measurement uncertainty has been many times smaller than the differences described here. The shortest day in that period fell on 5 July 2024, about 1.65 milliseconds shorter than 86,400 seconds.

Days more than a millisecond short occurred in that period in 2004 and 2005 (two in each year), and then in every year from 2020 to 2025. From 2021 to 2025 the average day was shorter than 86,400 seconds. 2026 is different: from 1 January to 15 September the average day was about 0.3 milliseconds longer, although over the same period in each of the five previous years it was shorter. No day in that time was more than a millisecond short.

For leap seconds, the direction matters. When days are shorter than 86,400 seconds, the UT1 minus UTC difference grows. If it approached plus 0.9 seconds, a second would have to be removed from UTC rather than added. Such a negative leap second has never been applied. An expert workshop organized in March 2025 by the Consultative Committee for Time and Frequency (CCTF) and the IERS estimated that its probability would rise quickly, reaching 30% by 2035. According to the 2026 resolution, a negative leap second poses a high risk of disruption to critical infrastructure that is largely unprepared, and preparing for it would cost industries that rely on time synchronization about as much as preparing for the millennium bug.

In mid-September 2026 the difference between UT1 and UTC was about minus 0.007 seconds, according to the IERS, far from the threshold in either direction.

The leap hour: what it really means

The term “leap hour” does not appear in the resolution. The document only sets an upper limit on the difference between UT1 and UTC: 3,600 seconds. It sets no date for any one-hour correction and no procedure for making one. The limit is meant to last for several centuries.

An hour sounds like a lot, but the resolution itself offers a point of reference. In many countries civil time already differs from local solar time by more than an hour because of how time zones were drawn, and daylight saving time regularly shifts clocks by an hour. In Poland, summer clocks show UTC plus two hours.

A second in true scale

A time difference is easy to turn into a distance. Relative to the Sun, Earth turns on average 15 degrees an hour, or 360 degrees a day. The equator's circumference, calculated from the 6,378.137 km equatorial radius given by NASA, is about 40,075 km. How people measured it long before satellites is the subject of our article How humanity measured the shape of Earth.

  • 1 millisecond, the order of magnitude of changes in day length, is about 46 cm at the equator.
  • 0.9 seconds, the old limit on the UT1 minus UTC difference, is about 420 m.
  • 1 second is about 464 m.
  • 1 hour, the new limit, is 15 degrees of longitude, or about 1,670 km at the equator.

That is how large the longitude error at the equator would be for someone fixing their position from the Sun who took UTC as Earth's rotation time. This is why anyone who needs precise rotation time uses the UT1 minus UTC correction. According to the resolution, it is available in satellite navigation messages and from services run by the IERS and national metrology institutes.

Why leap seconds troubled computers

The 2022 resolution states that jumps in UTC risk causing serious malfunctions in critical digital infrastructure, including satellite navigation, telecommunications and power transmission. Trouble hits software that assumes time only moves forward.

Cloudflare described one example. At midnight UTC on 1 January 2017, right after the last leap second, its DNS software measured a negative response time because the clock went back by a second. Some queries failed, and at peak the problem affected about 0.2% of DNS queries to Cloudflare.

That is why some companies “smear” the leap second instead of stepping the clock. Google has done so since 2008 and recommends spreading the second evenly over 24 hours, from noon to noon UTC, as a standard. The 2026 resolution notes, however, that the different methods used by service providers create timing inconsistencies between systems that do not conform to the SI unit.

What it changes in everyday life

You will not notice it day to day. Clocks will no longer receive an extra second, and official time in Poland, for example, will remain UTC plus one hour in winter and UTC plus two hours in summer, as set out in the Polish Official Time Act. Daylight saving time changes have nothing to do with this decision.

The gap between clocks and Earth's rotation will grow slowly, but it cannot exceed one hour, and according to the resolution that limit is enough for several centuries. The resolution gives examples of infrastructure that needs continuous time: transport, energy distribution, telecommunications and space projects.

Frequently asked questions

When was the last leap second?

On 31 December 2016 at 23:59:60 UTC. In Poland it was already 1 January 2017, at 00:59:60.

How many leap seconds have there been?

27. The first was inserted on 30 June 1972 and the last on 31 December 2016. All of them were positive, meaning each made the day one second longer.

Will there be another leap second?

The IERS has announced that there will be none at the end of December 2026. Leap seconds are inserted at the end of June or December, and the end of June 2027 falls after continuous UTC takes effect. Everything therefore suggests that the second added on 31 December 2016 was the last.

Is the leap hour the same as daylight saving time?

No. A daylight saving change shifts a country's official time by an hour, while UTC itself stays the same. The resolution sets no date for any one-hour correction to UTC.

Do I need to do anything with my phone or computer?

No. The decision concerns how the world's time scale is kept, not device settings.

Sources

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