A leap second is a one-second correction added to Coordinated Universal Time (UTC) to keep it within 0.9 seconds of Earth's actual rotation. On 15 October 2026 the General Conference on Weights and Measures (CGPM) voted to stop inserting leap seconds from 20 May 2027, letting UTC drift from Earth's rotation by up to one hour instead of one second.
On 15 October 2026, delegates meeting at the General Conference on Weights and Measures (CGPM) in Versailles, France, voted to stop adding leap seconds to Coordinated Universal Time (UTC), the time standard underlying clocks, satellites and computer networks worldwide. The vote, recorded as Resolution 5 of the 28th CGPM, sets 20 May 2027 — World Metrology Day — as the date UTC becomes a continuous timescale, free of the one-second adjustments used since 1972.
What a Leap Second Actually Is
UTC is built from two different ways of measuring time. International Atomic Time (TAI) ticks at a constant rate defined by atomic clocks. Universal Time (UT1) tracks Earth's actual rotation, which varies slightly because of winds, ocean currents, melting glaciers and movements inside the planet's core and mantle. UTC was designed as a compromise: it runs at the same rate as TAI but was occasionally nudged by a leap second so it never drifted more than 0.9 seconds from UT1.
The system began in 1972, when UTC was already 10 seconds behind TAI. Since then, 27 leap seconds have been inserted — always adding a second, never subtracting one — with the most recent on 31 December 2016. As a result, TAI has run 37 seconds ahead of UTC since January 2017.
Who Decides, and How
Scheduling leap seconds has been the job of the International Earth Rotation and Reference Systems Service (IERS) since 1 January 1988. The International Bureau of Weights and Measures (BIPM), based near Paris, combines readings from a global network of atomic clocks to produce TAI and, from it, UTC. When measurements showed UT1 and UTC approaching the 0.9-second limit, IERS announced a leap second roughly six months in advance, timed to the end of June or December.
Why Timekeepers Want to Scrap It
Leap seconds create a problem most software was never built to handle: a minute with 61 seconds, or a clock that briefly repeats itself. According to ABC News Australia, past leap seconds have been linked to outages at platforms including Cloudflare and Reddit, because the extra second violated assumptions built into timing code. The irregular, short-notice scheduling made it difficult for operators of satellite, telecom and financial systems to prepare.
A newer complication accelerated the push to drop leap seconds entirely. Earth's rotation has been speeding up since the mid-2010s, moving closer to the point where UTC might need a negative leap second — removing a second rather than adding one — for the first time ever. Darryl Veitch of the University of Technology Sydney told ABC News Australia that such an adjustment 'has never happened before, and therefore the software for that has never been tested.' Scientific American reported that engineers have warned such a scenario could resemble a 'mini Y2K' event for digital infrastructure, while Nick Stamatakos of the U.S. Naval Observatory said avoiding a negative leap second was 'a very big driver' behind widening UTC's tolerance.
After 30 years of debates, we are ready.
— Noël Dimarcq, CNRS senior scientist and BIPM time-and-frequency committee president, quoted by Scientific American
Two Resolutions, Five Years Apart
The decision was not made in a single step. At its 27th meeting in November 2022, the CGPM adopted Resolution 4, agreeing that the maximum permitted difference between UT1 and UTC would be increased 'in, or before, 2035.' The resolution asked the International Committee for Weights and Measures (CIPM) to propose a specific new tolerance, with a final decision due at the CGPM's next meeting in 2026.
That meeting took place in Versailles from 12 to 15 October 2026. On its final day, the CGPM adopted Resolution 5, fixing the new maximum allowed gap between UT1 and UTC at 3,600 seconds — one hour — and setting 20 May 2027 as the date the change takes effect, according to Scientific American and the published BIPM resolution text.
Leap second rules: before and after 20 May 2027
Aspect
Until 19 May 2027
From 20 May 2027
Maximum UT1–UTC drift allowed
0.9 seconds
3,600 seconds (1 hour)
Adjustment mechanism
Leap second (±1 second)
Leap hour (mechanism not yet designed)
Who announces it
IERS, about 6 months ahead
Still IERS, but not expected to be needed soon
Times used historically
27 leap seconds since 1972, last on 31 December 2016
None — threshold not expected for roughly 1,000 years
What Happens Next
Under the new rule, UTC will run at the same rate as atomic time without correction until the gap with Earth's rotation approaches one hour — a point ABC News Australia says is unlikely to arrive for roughly 1,000 years. If or when it does, any future 'leap hour' adjustment has not yet been designed; the 2026 resolution deliberately avoids specifying a replacement mechanism before one is needed. For most computer systems, the practical effect is removal, not addition: engineers can retire the special-case code written to handle leap seconds, and 31 December 2016 remains, for the foreseeable future, the last time a leap second was ever added to UTC.
Questions
What exactly is a leap second?
It is a one-second adjustment occasionally added to Coordinated Universal Time (UTC) to keep it within 0.9 seconds of UT1, the time scale based on Earth's actual rotation. Twenty-seven were inserted between 1972 and the most recent one on 31 December 2016, according to Wikipedia's summary of IERS records.
Why are leap seconds being abolished?
They break software that assumes every minute has exactly 60 seconds and that clocks never repeat or jump, and have been linked to past outages at platforms such as Cloudflare and Reddit, per ABC News Australia. A newer concern is that Earth's rotation has sped up since the mid-2010s, raising the risk of an untested negative leap second.
When does the leap second system actually end?
The CGPM's Resolution 5, adopted 15 October 2026 at its 28th meeting in Versailles, sets 20 May 2027 — World Metrology Day — as the date UTC stops using leap-second adjustments, according to Scientific American and the BIPM's resolution text.
What replaces the leap second?
Instead of frequent one-second corrections, UTC will be allowed to drift from UT1 by up to 3,600 seconds (one hour) before any adjustment is made. ABC News Australia reports that such a 'leap hour' is not expected to be needed for roughly 1,000 years.
Will my phone or computer clock change on 20 May 2027?
No single clock jump occurs on that date. The change simply removes the mechanism that could insert or remove a second in future; the last leap second actually applied remains 31 December 2016.
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Leap seconds, the one-second adjustments used since 1972 to keep clocks aligned with Earth's rotation, will stop entirely. The world's timekeeping authority, the International Bureau of Weights and Measures (BIPM), confirmed on 15 October 2026 that continuous UTC takes effect on 20 May 2027. The move, first agreed in principle in 2022, was accelerated after experts flagged a growing risk of a disruptive "negative" leap second.
