Leap Seconds Explained — Why the World Is Scrapping Them From May 2027

For most people, a clock reading '23:59:60' sounds like a glitch. Since 1972, it has happened 27 times on purpose. That extra tick — a leap second — has been the mechanism keeping the world's official clock in step with the slow, uneven spin of the planet. From 20 May 2027, it will stop happening.

What a leap second actually is

According to the Wikipedia entry on the subject, three time scales matter here. International Atomic Time (TAI) is counted purely by atomic clocks and never deviates. UT1 is time measured from Earth's actual rotation, which varies irregularly because of tidal friction, glacial rebound and other geophysical effects. Coordinated Universal Time (UTC) — the standard used for civil clocks, computers and GPS — is built from atomic seconds but has historically been nudged with leap seconds to stay within 0.9 seconds of UT1.

When UT1 and UTC drifted close to that limit, the International Earth Rotation and Reference Systems Service could schedule a leap second, inserted at the end of June or December. The last one was added on 31 December 2016. By 2017, TAI was running exactly 37 seconds ahead of UTC, per Wikipedia's summary of the system.

Why timekeepers decided to scrap the system

The push to abolish leap seconds dates back further than this year. The 27th General Conference on Weights and Measures (CGPM) passed a resolution in November 2022 directing that the maximum permitted gap between UT1 and UTC be widened before 2035, according to Wikipedia's account of that meeting.

The immediate trigger for finalizing the change was a technical problem nobody had faced before. Scientific American reports that Earth's rotation, which had been slowing through earlier decades, began accelerating in the mid-2010s — to the point that scientists projected a negative leap second, removing a second rather than adding one, might be needed around 2029. No negative leap second has ever been applied to any computer system, and engineers warned it could trigger failures comparable to the Y2K bug, Scientific American notes.

Euronews reporting on the same meeting adds that regular leap seconds have themselves already caused problems, having 'disrupted computer systems and online services' in the past, which combined with the negative-leap-second risk to push regulators toward a permanent fix rather than further patching.

The vote: Resolution 5 of the 28th CGPM

The decision was made official at the 28th meeting of the CGPM, held in Versailles, France, with the resolution passing by vote on 15 October 2026, according to Euronews and Scientific American. Scientific American describes the gathering as bringing together international scientists, diplomats and timekeepers. Euronews reports that the CGPM's membership currently stands at 69 member states and 34 associated states and economies.

The International Bureau of Weights and Measures (BIPM), the Paris-area intergovernmental body that compiles UTC by averaging atomic clock data from member states, published the resulting text as Resolution 5 of the 28th CGPM. Its operative provisions state plainly that 'continuous UTC will become effective on 20 May 2027' and that 'the maximum value for the difference |UT1-UTC| will be 3 600 seconds (1 hour).' The same BIPM text explains the reasoning: a negative leap second 'has not previously been applied, and it is considered to pose a high risk of causing anomalies and disruption to critical infrastructures.'

What changes, and what doesn't

From 20 May 2027, UTC will simply count forward as a continuous atomic time scale, with no insertions or deletions of seconds. The tolerance between UTC and astronomical time, previously capped at under one second, widens to as much as one hour under the new rule. Euronews and the BIPM resolution both note that experts do not expect the gap to actually reach that one-hour limit for a very long time — Scientific American cites an estimate of over 1,000 years, while Euronews reports experts saying it will be a considerable time, likely centuries, before any correction would actually be required.

The change mainly affects systems that have to translate between atomic time and civil time — satellite navigation, telecommunications networks, financial trading platforms and power-grid synchronization — all of which, per Euronews, stand to benefit from no longer having to handle irregular one-second insertions. Fields that depend on precise astronomical alignment, such as geodesy and some areas of astronomy, will instead need to track the growing UT1-UTC gap separately rather than relying on UTC to do it for them.

Ordinary clocks, phones and daily schedules will not visibly change on 20 May 2027. Euronews points out the shift is imperceptible to the public and distinct from recurring seasonal clock changes.

What is still to come

A separate measure adopted at the same Versailles meeting, Resolution 4 on the future definition of the second, lays groundwork for eventually redefining the second itself, with a decision expected to be presented at the 29th CGPM meeting in 2030, according to BIPM resolution materials. That is a distinct, longer-running project from the leap-second change and does not alter the 20 May 2027 timeline for continuous UTC.