October 15, 2026: The world has decided to get rid of leap seconds. The General Conference on Weights and Measures (CGPM) in Paris has approved a resolution to allow the difference between civil time (UTC) and solar time (UT1) to increase from 1 second to 1 hour.
What Is a Leap Second?
It’s an extra second that, every once in a while, is added to our clocks. Since the leap second system was introduced in 1972, there have been 27 of these additional seconds—most recently, at the end of December 31, 2016.
Why Were They Introduced?
To keep our clocks in sync with Earth’s rotation. Roughly speaking, Earth spins once every 24 hours with respect to the Sun. Its rotational speed is gradually slowing—but in an unpredictable and erratic way. Leap seconds ensure that civil time (UTC, which is based on the steady beat of atomic clocks) keeps within one second of solar time (UT1, which is essentially based on the position of the Sun in the sky).
What’s Happening This Week?
There’s a big meeting in Paris of the General Conference on Weights and Measures (CGPM), the policy-making body of the International Bureau of Weights and Measures (BIPM), where 69 member states agree on global standards for measurements. On the final day of the CGPM (October 15, 2026), the conference will vote on a resolution to change the rules for the world’s time standard: UTC.
What Does the Resolution Say?
To recap, the current rule is that UTC (civil time) has to stay within one second of UT1 (solar time)—meaning a leap second has to be added to UTC every few years or so. The resolution proposes that “the maximum value for the difference UT1-UTC will be 3600 seconds (1 hour), ensuring the long-term continuity for UTC for several centuries.”
What Does That Mean in Practice?
It means we won’t have to make any adjustments to our clocks—no more leap seconds—for a few thousand years. No further action will be needed until the difference between UTC and UT1 reaches one hour, at which point the world will have to decide what to do next.
So, Leap Hours Instead of Leap Seconds?
We put that question to Dr. Christian Bizouard at the International Earth Rotation and Reference System Service (IERS)—currently, he’s the person responsible for making official announcements about leap seconds. “It postpones the decision,” he tells us. “A leap hour would occur in the very, very remote future. A future generation will decide if they want to apply a leap hour, or maybe they won’t. We don’t know.”
What’s the Problem with Leap Seconds?
“When we apply leap seconds, there are always errors,” says Dr. Bizouard. And, these days, small errors can have big consequences. “When the leap second system was invented, society did not work with very, very small time resolution. But now there are many applications that need time resolution below one second, for example, financial systems. For that reason, changing UTC by one second is a problem.”
Hasn’t Earth’s Spin Been Speeding Up?
Yes, in recent years we’ve been reporting on an unexpected increase in the rate at which Earth spins. This raised the possibility that the next leap second might have to be a negative leap second: instead of adding a second to our clocks, we would make them skip a second instead. A negative leap second has never been tried before, and some fear it would cause widespread disruption.
When Will the Rule Change Happen?
The resolution proposes that the new system—which it calls “continuous UTC”—will come into effect on May 20, 2027.
Why the Rush?
A technical workshop held last year concluded that , following the recent unexpected increase in Earth’s rotational speed, there was a 30% probability that a negative leap second would be triggered within the next 10 years under the current rules. That’s a scenario many are keen to avoid.
Why May 20?
It’s World Metrology Day, held each year to celebrate metrology—the study of measurement.
Are We Removing the Link Between Time and the Sun?
Not according to Dr. Patrizia Tavella, the Director of the BIPM’s Time Department. “What is important is that we keep UTC related to the rotation of the Earth,” she says. “We want to have an international timekeeping which is related to the position of the Sun.” She points out that a one-hour drift between civil time and solar time is not much when compared to differences that already exist because of time zones and Daylight Saving Time.
When your clock doesn’t reflect solar timeIs This the Best Way Forward?
Finding a way to coordinate the steady beat of atomic time with the erratically changing speed of Earth’s rotation is a difficult problem that requires some kind of compromise. Dr. Tavella comments: “I am sure that if I ask you, will you agree with the solution that we propose at the end, you will say no, because maybe it would be better… Yes, we know! There are several ideas and several small details that would be important. But we have to solve this issue and find a decision.”
