Leap Seconds: What Changes When They End in 2035

·Published: 8 August 2026

A leap second is a one-second adjustment that keeps Coordinated Universal Time, or UTC, close to time based on the Earth’s rotation. The practice is due to change by or before 2035, when timekeepers will allow a larger difference between the two measures instead of correcting UTC whenever the gap approaches one second. For an ordinary UK clock user, almost nothing noticeable will happen: phones, laptops, radio-controlled clocks and network services should continue showing civil time as usual.

Why UTC sometimes needs an extra second

Modern timekeeping relies on atomic clocks, which measure exceptionally stable transitions within atoms. Their combined output provides International Atomic Time, known as TAI. UTC runs at the same rate as TAI but includes whole-second adjustments intended to keep it reasonably close to UT1, a measure linked to the Earth’s angle of rotation. That link matters because the position of the Sun in the sky follows the rotating Earth, not an atomic clock in a laboratory.

The Earth does not rotate with the regularity of an atomic time scale. Its speed changes slightly under influences that include movements in the atmosphere and oceans, interactions between the Earth and Moon, and processes inside the planet. The difference is tiny from one day to the next, but it accumulates. Under the current arrangement, a leap second may be introduced when the value of UT1 minus UTC is predicted to approach 0.9 seconds. This keeps clock time and rotation-based time within a narrow range without making people adjust their watches manually.

What happens during a leap second

A positive leap second creates an unusual final minute in UTC. Instead of moving directly from 11:59:59 pm to 12:00:00 am, the sequence includes 11:59:60 pm. The International Earth Rotation and Reference Systems Service determines when an adjustment is needed, while time-distribution systems pass the information to computers and other equipment. A negative leap second would remove a second, although no negative leap second has yet been used. In Britain, the displayed local time would depend on the season, because the UK alternates between Greenwich Mean Time and British Summer Time, but the underlying UTC event would still be global.

Computers often represent time as a steadily increasing count, and many programs assume every minute contains exactly 60 seconds. An inserted second can therefore produce a repeated timestamp, an unfamiliar value ending in “60”, or an apparent step backwards. Those possibilities can confuse databases, timers and distributed systems that need to establish which event happened first. Some operators avoid the abrupt step through a “leap smear”: their clocks run imperceptibly slower before and after the event, spreading the additional second across a longer period.

Past adjustments exposed real software faults. A leap second in 2012 contributed to a Reddit interruption lasting roughly 30 to 40 minutes after timer behaviour drove heavy processor activity. At the start of 2017, Cloudflare experienced failures in a portion of its DNS service because code calculated a negative time interval when the clock appeared to move backwards. The incidents were not caused by people’s wall clocks being one second wrong. They arose because software had been written on the assumption that machine time always advanced smoothly.

What the 2035 decision actually changes

In November 2022, the General Conference on Weights and Measures adopted Resolution 4 on the future of UTC. It decided that the permitted maximum difference between UT1 and UTC would be increased by or before 2035. The practical aim is to make UTC continuous for a much longer period, avoiding the frequent one-second discontinuities that create risks for telecommunications, navigation and other precisely synchronised infrastructure. Further technical work determines the new tolerance and implementation details, so describing the decision simply as abolishing time correction altogether misses part of the picture.

UTC will gradually move farther from rotation-based time once routine leap seconds stop, but the divergence will be very small on the scale of everyday life. Noon on a clock already differs from the instant when the Sun reaches its highest point because of longitude, the time zone and seasonal clock changes. A growing fraction of a minute will not alter when your 7:15 am alarm sounds, whether a train is timetabled for 8:12 am, or when a meeting begins. Far in the future, time authorities may use a larger adjustment, but the 2022 resolution asks for a system that can provide continuity for at least a century.

Do you need to change any clocks?

You should not need to do anything special. Connected phones and computers obtain time from synchronisation services, while broadcasters and other infrastructure providers distribute civil time without asking each user to handle leap seconds. A kitchen clock or wristwatch that is adjusted by hand is unlikely to reveal a one-second difference anyway. The main work falls to metrology bodies, network operators and developers of systems that depend on exact timestamps. For most people in the UK, the change means fewer hidden technical hazards behind the familiar time displayed on a screen, not a new way of reading the clock.