World's timekeepers vote to kill the leap second — and untether clocks from Earth's rotation

For the first time in more than half a century, the world's clocks may stop following the Earth. Meeting Thursday in Versailles, the 69 member states of the International Bureau of Weights and Measures — known by its French acronym BIPM — are voting on whether to abolish the leap second, the occasional extra tick added to global time since 1972 to keep atomic clocks in step with the planet's rotation.

The vote is widely expected to pass. If it does, the change would take effect on May 20, 2027 — World Metrology Day — accelerating a phase-out that the BIPM had previously set for 2035.

The leap second exists because of a fundamental mismatch: the atomic clocks that define Coordinated Universal Time (UTC) are extraordinarily precise, while the Earth is not. The planet wobbles and its rotation gradually slows, meaning a day is never exactly 24 hours. Since 1972, timekeepers have bridged that gap by inserting an extra second — 27 of them in total — at the end of June 30 or December 31, producing the peculiar timestamp 23:59:60 before the clock rolls over to midnight. The last one was added on December 31, 2016.

The case for abolition is rooted in a decade of documented failures. In 2012, the addition of a leap second crashed LinkedIn and Reddit and collapsed ticket bookings for Australian airline Qantas. Three years later, the Australian Securities and Investments Commission issued a formal warning about the risk. In July 2022, engineers at Meta described every leap second as 'a major source of pain for people who manage hardware infrastructures' — a sentiment shared across financial markets and data centres that depend on timing precise to the millisecond.

Every leap second is a major source of pain for people who manage hardware infrastructures.— Engineers, Meta

A newer and more unsettling problem has sharpened the urgency. While the Earth's rotation has been slowing for billions of years under the Moon's gravitational influence, it has actually been speeding up over the past decade. That raises the prospect of something that has never happened before: a negative leap second — a moment removed from the clock rather than added to it.

Unusually, the Earth's rotation has accelerated in recent years, creating the additional possibility that a negative leap second may be required for the first time. Current estimates suggest around a 30 percent probability of this occurring within the next decade.— Setnam Shemar, Principal Scientist, National Physical Laboratory

Removing a second from global timekeeping systems — systems built only to add — is a largely untested operation, and the risk of cascading failures is considered far greater than anything a positive leap second has caused.

The leading proposal before the BIPM would not simply abandon the problem — it would defer it on a vastly longer timescale. Instead of correcting the drift between UTC and astronomical time (known as UT1) whenever it exceeds 0.9 seconds, the new tolerance would be up to one full hour. When that threshold is eventually crossed, a 'leap hour' would be inserted. Given that 27 seconds accumulated over 50 years, that point is centuries or even millennia away.

Previously, leap seconds were added to UTC to keep it within 0.9 seconds of UT1. From 20 May 2027 a difference of up to 1 hour will be allowed before intervention is needed, ensuring the continuity of UTC for several centuries or more.— Setnam Shemar, Principal Scientist, National Physical Laboratory

For most people, day-to-day life will not change. Scientists say human routines will be unaffected and the software glitches and network failures associated with leap seconds will be eliminated. But the long-term arithmetic is stark: by 2100, clocks will run 54 seconds ahead of the Earth's actual position. Over two millennia, that gap widens to several hours — meaning sunrise and sunset would feel increasingly out of sync with the time displayed on a clock.

The relationship between UTC and UT1 is important for some applications, such as astronomy and celestial navigation. The decision would not sever that link: UT1 will continue to be monitored and its difference from UTC will continue to be disseminated through established international services.— Setnam Shemar, Principal Scientist, National Physical Laboratory

The Versailles meeting is also considering two other proposals. One would redefine the second itself: optical atomic clocks — which measure the vibration of atoms at light frequencies — are now more precise than the microwave-based atomic clocks that have defined the second for more than seven decades, and there is a push to make optical frequency the new standard. A third proposal would align Earth's timekeeping with a new lunar time standard being developed by NASA.

Not all member states are enthusiastic. Russia has pushed to delay any phase-out until 2040, because its GLONASS satellite-navigation system incorporates leap seconds directly and would require significant updates. Despite that objection, the resolution is expected to pass on a majority vote, according to IFLScience, because most national delegates have supported ending leap seconds for several years.

Any proposals accepted Thursday are expected to take effect on May 20, 2027. The leap second — a fixture of global timekeeping for 55 years — would then become a relic, replaced eventually by a correction so large and so infrequent that no one alive today is likely to see it.

Why it matters — Abolishing the leap second would eliminate a recurring source of tech failures for financial markets and internet infrastructure while quietly starting a centuries-long drift between the clocks that run modern life and the planet those clocks are supposed to track.

Reported by aljazeera.com, iflscience.com, swedenherald.com