The way the world keeps time is about to change. An international body of scientists, diplomats and timekeepers has voted to retire the leap second — the small correction that has kept official time in step with the Earth's rotation for more than five decades — and replace it with a "leap hour" system that lets clocks drift far further from astronomical time.
The vote
The vote took place at the General Conference on Weights and Measures (CGPM) in Versailles, France, and won by a majority. Under the new system, Coordinated Universal Time (UTC) — the global standard that governs computers, phones, aircraft, satellites and navigation systems such as GPS — will be allowed to drift from Earth's rotation (UT1) by up to one hour, or 3,600 seconds, instead of being held within a single second.
"After 30 years of debates, we are ready," said Noel Dimarcq, a senior scientist at France's CNRS and president of the International Bureau of Weights and Measures' consultative committee on time and frequency, just before the vote. The change takes effect on May 20, 2027, which is World Metrology Day.
Why the change
Shortly after UTC was established in 1960, scientists noticed that the Earth's rotation was slowing slightly and falling behind atomic clocks, so the first leap second was added in 1972. Since then, 27 leap seconds have been inserted to keep UTC aligned with the planet's spin.
But the irregular corrections have become a headache for digital infrastructure. Leap seconds can crash websites and systems — Reddit famously went down when one was added in 2012 — and tech companies such as Google and Meta resort to "smearing" the extra second across longer periods. More recently, the Earth's spin began accelerating, raising the prospect of the first-ever negative leap second — a one-second rollback engineers compared to a "mini Y2K" event, since no software had ever been tested for it.
Avoiding that scenario was a major driver of the decision, said Nick Stamatakos of the US Naval Observatory. Scientists estimate a leap hour will not be needed for more than 1,000 years, effectively ending the routine link between UTC and the sky. For most people, the shift will be invisible — but it means civilisation will keep drifting further from the oldest way of telling time: looking up.
Sources:
- Scientists vote to end leap seconds, redefining how the world keeps time — Scientific American
- It's official: timekeepers have gotten rid of the leap second — and introduced the leap hour instead — Smithsonian Magazine
The vote
The vote took place at the General Conference on Weights and Measures (CGPM) in Versailles, France, and won by a majority. Under the new system, Coordinated Universal Time (UTC) — the global standard that governs computers, phones, aircraft, satellites and navigation systems such as GPS — will be allowed to drift from Earth's rotation (UT1) by up to one hour, or 3,600 seconds, instead of being held within a single second.
"After 30 years of debates, we are ready," said Noel Dimarcq, a senior scientist at France's CNRS and president of the International Bureau of Weights and Measures' consultative committee on time and frequency, just before the vote. The change takes effect on May 20, 2027, which is World Metrology Day.
Why the change
Shortly after UTC was established in 1960, scientists noticed that the Earth's rotation was slowing slightly and falling behind atomic clocks, so the first leap second was added in 1972. Since then, 27 leap seconds have been inserted to keep UTC aligned with the planet's spin.
But the irregular corrections have become a headache for digital infrastructure. Leap seconds can crash websites and systems — Reddit famously went down when one was added in 2012 — and tech companies such as Google and Meta resort to "smearing" the extra second across longer periods. More recently, the Earth's spin began accelerating, raising the prospect of the first-ever negative leap second — a one-second rollback engineers compared to a "mini Y2K" event, since no software had ever been tested for it.
Avoiding that scenario was a major driver of the decision, said Nick Stamatakos of the US Naval Observatory. Scientists estimate a leap hour will not be needed for more than 1,000 years, effectively ending the routine link between UTC and the sky. For most people, the shift will be invisible — but it means civilisation will keep drifting further from the oldest way of telling time: looking up.
Sources:
- Scientists vote to end leap seconds, redefining how the world keeps time — Scientific American
- It's official: timekeepers have gotten rid of the leap second — and introduced the leap hour instead — Smithsonian Magazine