World Timekeepers End Leap Seconds From 20 May 2027

The world’s official time will stop adding leap seconds. Member states of the General Conference on Weights and Measures (CGPM) have decided that “continuous UTC will become effective on 20 May 2027,” ending the occasional one-second adjustments that keep clocks in step with the Earth’s rotation.

The decision is set out in Resolution 5 of the CGPM’s 2026 meeting, published by the International Bureau of Weights and Measures (BIPM). The BIPM notes that the official version of the resolution is the French text.

What changes

Coordinated Universal Time, or UTC, is the time scale behind civil time in most countries and the clocks on phones, computers and networks. It is based on atomic clocks, which are extremely regular. The Earth’s rotation is not, so UTC has been kept within a second of astronomical time, known as UT1, by inserting a leap second when needed.

Under the new resolution, that stops. The CGPM decided that “the maximum value for the difference |UT1-UTC| will be 3 600 seconds (1 hour), ensuring the long term continuity for UTC for several centuries.” In other words, atomic time and the Earth’s rotation will be allowed to drift apart by up to an hour before any correction is considered.

The CGPM had already decided in 2022 that the limit would be increased “in, or before, 2035.” This resolution sets the date and the new limit.

Why leap seconds are being dropped

The resolution says industrial users and national critical infrastructure, “for example transportation, energy distribution, telecommunication, and space projects require UTC to be continuous without any future adjustment.”

Leap seconds have caused problems because computer systems often do not expect a minute with 61 seconds. The resolution also says that the various methods large service providers use to smooth over leap seconds in their servers “leads to inconsistency in timing between different systems.”

The risk of a negative leap second

A bigger worry is a leap second in the other direction. So far, every leap second has added time. But a workshop held by the Consultative Committee for Time and Frequency and the International Earth Rotation and Reference Systems Service in March 2025 estimated that the probability of a negative leap second, which would remove a second, “will increase rapidly in the near future, reaching a 30 % probability by 2035,” according to the resolution.

A negative leap second “has not previously been applied, and it is considered to pose a high risk of causing anomalies and disruption to critical infrastructures that are largely unprepared,” the resolution says. It adds that preparing for one would require investment by industries that rely on time synchronisation, “whose amount is estimated to be similar to their preparations for the millennium bug.”

The CGPM also warned that delay could push some organisations to switch to other time scales, such as satellite navigation (GNSS) time instead of UTC, “seriously compromising the concept of international standardization in metrology.”

A transition for radio time signals

Some older equipment that receives time by radio cannot handle a larger gap between UT1 and UTC. The resolution notes that the International Telecommunication Union’s Resolution 655 (REV.WRC-23) allows a transition period for radio dissemination “until 2035, but no later than 2040, in cases where existing equipment cannot be replaced earlier.”

People who need astronomical time, such as some astronomers and navigators, can still get the UT1-UTC difference from satellite navigation messages and online services, the resolution says.

What’s still unknown

The resolution does not set out what, if anything, will happen when the one-hour limit is eventually approached. It is not yet clear how quickly every system and radio service will be updated before 2027.

What to watch

Watch for guidance from the BIPM, the IERS and national metrology institutes on implementation, and for software and network operators confirming that their systems are ready.

What this means

No more leap seconds. UTC becomes continuous on 20 May 2027.
Up to an hour of drift. Clocks may drift up to 3,600 seconds from Earth’s rotation.
Infrastructure risk reduced. A risky first negative leap second is avoided.