Why the world is scrapping the leap second for a once-in-a-millennium "leap hour"

Coordinated Universal Time, the reference clock behind phone networks, satellites, stock exchanges and the internet, is being redefined. Member states of the General Conference on Weights and Measures (CGPM), the policy body of the International Bureau of Weights and Measures (BIPM), voted on 15 October 2026 at its meeting in Versailles, France, to stop adding leap seconds to UTC, according to reporting by eldiario.es and the BIPM's own draft resolution for the meeting.

What a leap second is, and why it exists

UTC is built from two different kinds of time. International Atomic Time (TAI) is counted by atomic clocks and never varies. UT1, by contrast, follows the Earth's actual rotation, which speeds up and slows down slightly because of tides, melting ice, winds and movements inside the planet's core, according to Wikipedia's entry on the leap second. Since 1972, the leap second has been the tool used to keep UTC's date and time of day aligned with UT1 while still ticking in SI seconds, the same unit used by atomic clocks.

The International Earth Rotation and Reference Systems Service (IERS) has decided when to add a leap second, normally giving six months' notice through a bulletin, whenever the UT1-UTC gap approached 0.9 seconds. Twenty-seven leap seconds have been inserted since the system began, most recently on 31 December 2016, leaving UTC 37 seconds behind TAI, per Wikipedia.

Why timekeepers decided to act now

The driver behind the change is a reversal in Earth's behaviour. For decades the planet's rotation was slowing, so seconds were added. More recently it has been spinning marginally faster. Scientific American reported that a negative leap second — subtracting a second rather than adding one, something that has never been done and was never built into most software — could be needed as early as 2029. Eldiario.es, citing a Spanish metrology official, put the same risk at roughly two decades away; the two sources therefore differ on the exact timing, though both describe it as a near-term possibility.

Nick Stamatakos, head of the Earth Orientation Department at the U.S. Naval Observatory, told Scientific American that avoiding a negative leap second was "a very big driver to open the window between UTC and UT1." Scientific American reported that engineers had warned a negative leap second could trigger failures across systems never tested for it, comparing the risk to a smaller version of the Y2K bug.

What changes, and when

Under the resolution, UTC becomes continuous from 20 May 2027, chosen because it is World Metrology Day. Instead of a maximum 0.9-second gap triggering a leap second every few years, the new rule allows UT1 and UTC to drift apart by up to 3,600 seconds — one hour — before any correction, a figure the BIPM's draft resolution and Scientific American both cite. Because Earth's rotation changes so slowly, scientists quoted by Scientific American and eldiario.es expect that a full leap hour will not be needed for roughly a thousand years, and eldiario.es reports that no UTC correction at all is expected for several centuries.

Spain's delegation supported the change, according to eldiario.es, and the vote passed by majority among the BIPM's member states.

Why past leap seconds caused technical trouble

Leap seconds have a track record of breaking software. Wikipedia's account of the practice lists outages at Reddit, Mozilla, Qantas and on Linux systems after the 2012 leap second, and disruptions at Twitter, Instagram, Netflix and Amazon following the one added in 2015. The problems typically stemmed from how operating systems, GPS receivers, network time protocol software and airline booking systems handled the extra, non-standard second, rather than from the length of the adjustment itself.

What it means for ordinary technology

For most people, the change will be invisible: clocks on phones and computers will keep showing the same time as before. The effect is on the software and hardware that previously had to account for an occasional extra or missing second — satellite navigation, telecom networks, financial trading systems and power-grid timing among them, areas eldiario.es and Scientific American both note were flagged as at risk from an untested negative leap second. By fixing UTC as continuous, engineers no longer need to code for that rare, disruptive event, even though a much larger one-hour adjustment remains possible in the very long term.