The key facts
- From 20 May 2027, UTC will run continuously, without leap seconds; in total, 27 have been added since 1972, the last one at the end of 2016.
- The decision was taken on 15 October 2026 at the 28th General Conference on Weights and Measures in Versailles.
- The divergence between UTC and astronomical time is allowed to reach 3600 seconds (one hour); experts consider the measure mostly precautionary.
Abolition of the leap second
The world's official time standard will stop adding leap seconds from 20 May 2027. The General Conference on Weights and Measures (CGPM) took this decision on Thursday, 15 October 2026, at its 28th meeting in Versailles, where delegates discussed the future of global timekeeping. Until now, leap seconds were periodically added so that atomic time would correspond to the Earth's rotation; from 20 May 2027, Coordinated Universal Time (UTC) will run continuously, without interruptions.
Why the adjustments were needed and why they are being abandoned
The previous rules were meant to reconcile two ways of measuring time: UTC relies on highly accurate atomic clocks, while astronomical time depends on the Earth's rotation, whose speed changes slightly. To keep the gap between the scales small, 27 leap seconds have been added since 1972; the last one at the end of 2016. Because of this, a minute in exceptional cases lasted 61 seconds. For people, the adjustments are almost imperceptible, but they can cause technical problems: computer systems, telecommunications networks, satellite navigation and energy infrastructure depend on accurate and reliable time synchronisation, and past leap seconds have already disrupted computer systems and online services.
The role of changes in the Earth's rotation
One of the reasons for the decision was the evolution of the Earth's rotation and the new risks of a 'negative' leap second associated with it. Over very long periods, tidal forces slow the planet down, but in recent years it has at times been spinning slightly faster. This created the possibility that, for the first time, a so-called negative leap second would be required — removing, rather than adding, a second. Since this has never been done, it is unclear how reliably technical systems would cope with it. Therefore, the international expert community decided to abolish the current rules ahead of schedule. Under the new rules, the gap between UTC and astronomical time will be able to grow and could potentially reach 3600 seconds, that is, one hour. In the experts' view, considerable time will pass before such a gap actually requires correction, so the measure is mostly precautionary.
About the conference and its participants
The CGPM brings together representatives of member states as well as associate states and economic territories and usually meets once every four years. According to the International Bureau of Weights and Measures (BIPM), it comprises 69 member states and 34 associate states and economies. In Europe these include, among others, Germany, Austria, France, Italy, Spain, Switzerland, the United Kingdom, Poland and Ukraine; the Americas are represented by the USA, Canada, Brazil, Argentina, Mexico and Peru; Asia by China, Japan, India, South Korea, Indonesia and Thailand; Africa by South Africa, Egypt, Morocco, Tunisia and Kenya. Australia and New Zealand are also represented.
What this means for the UK and for the exact sciences
For everyday life in the UK, the decision will bring no immediate changes. The abolition of the leap second is a technical adjustment to the international time standard and should not be confused with the familiar switch to daylight saving and winter time: according to the GOV.UK Clock Change Guide, the clocks will go back one hour on Sunday, 25 October 2026, at 2:00. Local time in the country will continue to follow Greenwich Mean Time (GMT, UTC+0 in winter) and British Summer Time (BST, UTC+1 in summer); seasonal clock changes are unaffected. The changes will have a greater impact on specialised fields that require precise reference to the Earth's rotation — astronomy, geodesy (the science of measuring and mapping the Earth, determining its shape, size and gravitational field) and some satellite applications. For these disciplines, precise knowledge of the Earth's rotation will remain critical: since leap seconds will no longer link global time to the rotation, they will have to separately account, where necessary, for the growing gap between clock time and astronomical time. Overall, the aim of the new rules is to make international timekeeping simpler and more reliable in the long term: digital systems will no longer have to cope with the insertion of extra seconds at irregular intervals.
