Meeting in Versailles, the General Conference on Weights and Measures has decided that Coordinated Universal Time will stop adding leap seconds from 20 May 2027. Atomic time and time based on Earth’s rotation will be allowed to drift up to an hour apart, ending a 54-year practice that has repeatedly disrupted computer systems. The same meeting began work on lunar time and on a new definition of the second.
For more than half a century, the world’s official time has been kept in step with the rotating Earth by an occasional one-second adjustment. On Thursday, 15 October, the countries that govern international measurement voted to stop.
At its 28th meeting in Versailles, the General Conference on Weights and Measures (CGPM) adopted Resolution 5, “On the technical actions needed to ensure the continuity of UTC”. According to the text published by the International Bureau of Weights and Measures (BIPM), continuous UTC will take effect on 20 May 2027, and the maximum permitted difference between UTC and UT1, the time scale based on Earth’s rotation, will be 3,600 seconds, or one hour. The BIPM says the new tolerance is intended to keep UTC continuous for several centuries.
The decision was not unanimous, Nature reported; it needed only a majority. “This is a historical moment, a decisive step for UTC,” said Noël Dimarcq of the Côte d’Azur Observatory in Nice, president of the CGPM’s Consultative Committee for Time and Frequency, who introduced the resolution. Georgette Macdonald of the National Research Council of Canada said the decision “will ensure the continuity and relevance of UTC for centuries to come.”
Two kinds of time
The problem is that the world runs on two clocks. Atomic time is extraordinarily stable: since 1967 the second has been defined by the frequency of radiation from caesium-133 atoms. Astronomical time follows Earth’s rotation, which is irregular. Tides slow the planet over the long term, while movements in the core, oceans, atmosphere and ice cause it to speed up or slow down unpredictably over years and decades.
Coordinated Universal Time (UTC), introduced in its current form in 1972, is a compromise. It ticks at the atomic rate but is kept within 0.9 seconds of UT1 by inserting a leap second when needed. The International Earth Rotation and Reference Systems Service announces them, typically six months ahead. Twenty-seven leap seconds have been added since 1972, the most recent at the end of 2016.
Why leap seconds became a problem
A minute with 61 seconds is awkward for computers. Leap seconds have caused failures in software that assumes every minute has 60 seconds. In 2012 the leap second disrupted several major websites, and on 1 January 2017 Cloudflare reported failures in its DNS service. Large technology companies now “smear” the extra second across hours, but they do so in different ways, so their clocks can briefly disagree.
A further risk has appeared. In recent years Earth’s rotation has been running slightly fast, raising the prospect of the first negative leap second, a minute with only 59 seconds. No such adjustment has ever been made, and much software has never been tested for it. Metrologists have warned that this could be more disruptive than the positive leap seconds the world has grown used to.
Satellite navigation adds complexity. GPS time does not include leap seconds and is now 18 seconds ahead of UTC, so receivers must apply the offset. Russia’s GLONASS is tied to UTC, and Russia has historically been cautious about abolition; secondary reports suggest it opposed this resolution, but NSH has not seen an official record of the vote.
From 2035 to 2027
The direction of travel was set in 2022, when the 27th CGPM resolved that the UT1–UTC tolerance would be increased in or before 2035. The new resolution brings that forward to 2027 and fixes the tolerance at one hour. At the current rate of divergence, an hour’s difference is unlikely to build up for many centuries. What happens then, whether a “leap hour” or some other adjustment, is left to future generations. Some critics have argued that this merely postpones the decision.
What changes in practice
For most people, nothing visible will change: clocks, phones and watches will keep showing the same time. The change matters to the operators of systems that distribute time. After May 2027, the International Earth Rotation and Reference Systems Service will no longer schedule leap seconds, and software will no longer need special handling for a 61-second minute. Astronomers, satellite operators and others who need the Earth’s exact orientation already use published values of the UT1–UTC difference, and they will continue to do so, with a gradually growing correction.
Lunar time and a new second
The conference also looked further ahead. Resolution 6 asks space agencies to document how their lunar time scales trace back to UTC, and asks the CIPM and BIPM to prepare to coordinate a lunar reference time. This is a pressing question as the United States, China, India and others plan sustained lunar activity, because clocks on the Moon run slightly faster than on Earth. Resolution 4 asks the International Committee for Weights and Measures to bring a proposal for redefining the second, based on optical atomic clocks, to the 29th CGPM in 2030. The best optical clocks are already about 100 times more accurate than the caesium standard.
What it means for India
Indian Standard Time is maintained by CSIR-National Physical Laboratory (NPL) in New Delhi, which contributes to the international atomic time scale and disseminates IST. Because IST is defined as UTC plus five hours thirty minutes, it will simply inherit the change. Telecom networks, power-grid synchronisation, stock exchanges, banking systems and India’s NavIC navigation system all depend on accurate time, and all have had to handle leap seconds. Removing them should reduce operational risk, but systems will need to be checked to ensure they do not insert a leap second after May 2027. NSH has not found a statement of India’s position in the vote.
