World timekeepers vote to scrap the leap second and replace it with a leap hour

After roughly 30 years of argument, the world's official timekeepers voted today, Oct. 15, to kill the leap second.

Delegates from 69 member states cast their votes at the General Conference on Weights and Measures (CGPM) in Versailles, France. The conference is run by the International Bureau of Weights and Measures (BIPM), which publishes Coordinated Universal Time, or UTC. The change is set to take effect in May 2027.

"After 30 years of debates, we are ready," said Noël Dimarcq, a senior scientist at France's CNRS and president of the BIPM's Consultative Committee for Time and Frequency, just before the vote.

What changes

Since 1972, timekeepers have added 27 leap seconds to UTC to keep it within one second of time based on Earth's rotation, known as UT1. The last one came at the end of 2016.

Under the new rules, the allowed gap between UTC and UT1 widens to one hour, or 3,600 seconds. A "leap hour" would only be applied if the two drift that far apart.

That is not expected soon. Estimates of the timing of the first leap hour range from centuries to thousands of years, depending on how Earth's spin behaves.

Why the leap second became a problem

UTC is built from a weighted average of atomic clocks around the world. Those clocks run steadily. Earth does not. Its rotation shifts with the pull of the Moon, ocean currents, winds, glacier melt, volcanoes and movement deep in the planet's interior, University of Tasmania geodesist Lucia McCallum told ABC Science.

For most of the leap-second era, Earth's rotation was slowing, so clocks needed an extra second. Since the mid-2010s, the spin has sped up. Scientists have projected that a negative leap second, which would skip a second and push the world's clocks backward in effect, might be needed as early as 2029.

Engineers compare that scenario to a "mini Y2K." Nick Stamatakos, head of the Earth Orientation Department at the U.S. Naval Observatory, which manages the country's GPS timing, said avoiding negative leap seconds was "a very big driver" behind widening the window.

Even the positive ones have caused damage. A leap second in 2012 knocked out Reddit and LinkedIn and delayed Qantas flights. Cloudflare suffered brief outages after the 2016 insertion. Meta, Google and Amazon have long lobbied against the practice, calling it "a major source of pain."

Google went a different way back in 2008. Rather than risk a one-step insertion, its time servers drifted from UTC for 20 hours, smearing the extra second in increments too small for its systems to notice.

Darryl Veitch, a computer networking researcher at the University of Technology Sydney, said leap seconds break basic software assumptions: that time only moves forward and that a minute has 60 seconds.

The case for keeping the link to Earth

The vote does not cut the tie to the planet entirely, according to its backers. Dr. Setnam Shemar, principal scientist in the time and frequency department at the UK's National Physical Laboratory, sat on the expert committee that proposed the leap hour. He said it "will reduce the risks to the systems and national infrastructure that depend on accurate time while still preserving the fundamental link between our global time system and the rotation of the Earth."

Shemar compared it to daylight saving time, an hour-long shift that people already handle twice a year.

There is a tradeoff, however. Jeff Sherman, a group leader in the National Institute of Standards and Technology's Time and Frequency Division in Boulder, Colorado, put the choice this way: decide whether the most important thing is a sense of time given by objects in the sky above a place like Greenwich, "or you decide to follow what you believe are more stable, laboratory, man-made clocks." The vote picks the lab clocks.

The central argument for ending leap seconds is that they endanger computing systems. The Independent noted that the underlying risk was to having a single globally accepted standard of time. Google's smearing shows large operators were already working around UTC rather than with it.

What comes next

The leap second was meant to keep clock time tied to the sun overhead. Astronomers and navigation systems still need Earth-rotation data, and that is still measured, including by telescopes tracking distant black holes, as McCallum described. What changes is that civil time will no longer be nudged to match it every few years.

The open question is the far future. A leap hour, whenever it comes, will be a one-hour jump for the whole planet's infrastructure. No one alive will have to handle it, and the BIPM has not set out how that distant correction would be carried out. The immediate deadline is May 2027, when UTC is scheduled to go continuous.