The world’s official timekeeping system is preparing for a historic change after international delegates voted to phase out the leap second, a tiny adjustment introduced to keep atomic clocks aligned with Earth’s irregular rotation.
The decision was approved on October 15, 2026, at the General Conference on Weights and Measures in Versailles, France. Under the new arrangement, continuous Coordinated Universal Time (UTC) will take effect on May 20, 2027, allowing the difference between atomic time and Earth’s rotational time to grow to as much as one hour before a major correction is required.
The change aims to protect computer networks, satellite navigation, telecommunications and other systems that depend on precisely synchronised clocks. Although the adjustment marks a major shift in how international time is maintained, everyday clocks will not suddenly gain or lose an hour in 2027.
Why do we need leap seconds?
Most people think of a day as exactly 24 hours, but Earth’s rotation does not follow a perfectly uniform schedule. Gravitational forces, movements within the planet’s core, ocean circulation, and atmospheric changes can all influence how quickly Earth spins.
Atomic clocks, meanwhile, measure time using extremely stable atomic frequencies. Their precision makes them essential for modern technology, but atomic time can gradually drift away from the time measured by Earth’s rotation.
To keep the two systems sufficiently close, international timekeepers introduced the leap second in 1972. When the gap becomes too large, an additional second can be inserted into UTC, generally at the end of June or December.
Since the system began, 27 leap seconds have been added. The most recent occurred on December 31, 2016.
Why are scientists ending the leap second?
The main concern is that leap seconds can disrupt computer systems that assume time progresses in a continuous and predictable way.
Adding an extra second can create unexpected problems in software, databases, communication networks and other infrastructure that relies on precise timestamps. Companies operating large digital systems must prepare for the adjustment, test their equipment and ensure that different systems handle the change consistently.
The risks became apparent in 2012, when leap-second-related software problems contributed to outages affecting websites including Reddit and LinkedIn. Qantas also experienced disruption to its booking systems, leading to delays and manual workarounds.
Technology companies have since warned that leap seconds create recurring engineering challenges. Meta described them in 2022 as a major source of difficulty for teams managing hardware infrastructure.
The concern is becoming more urgent because scientists have also considered the possibility of a negative leap second. Such an adjustment would remove a second rather than add one, creating a scenario for which many systems have never been tested.
What did timekeepers decide in Versailles?
Delegates from 69 countries voted to introduce a more continuous form of UTC during the October 2026 meeting.
Under the new arrangement, the maximum permitted difference between UTC and Earth’s rotational time, known as UT1, will increase to 3,600 seconds, or one hour. Continuous UTC is scheduled to take effect on May 20, 2027.
This means timekeepers will no longer need to insert leap seconds whenever the difference approaches the current limit. Instead, atomic time can continue without these periodic one-second adjustments while scientists keep measuring Earth’s rotation separately.
The approach is sometimes described as replacing the leap second with a “leap hour.” That phrase refers to the much larger adjustment that could eventually become necessary, not an hour being added to clocks next year.
Because the gap accumulates gradually, a full-hour correction is not expected for thousands of years under current projections.
Will the change affect everyday life?
For most people, the answer is no, at least in the immediate future.
Watches, smartphones and ordinary clocks will continue displaying time as usual. The duration of a second will not change, and countries will not need to alter their time zones simply because the leap-second system is being phased out.
The main benefits will be felt in sectors that require highly accurate timing, including satellite navigation, telecommunications, energy distribution, financial transactions and scientific research.
There will still be a difference between the time shown by civil clocks and the time associated with the Sun’s position in the sky. However, that difference will remain manageable for an extremely long period before a large adjustment becomes necessary.
Scientists will also continue monitoring Earth’s rotation and publishing the information needed by astronomers, navigation systems and other specialist users.
Why could a negative leap second have caused problems?
Earth’s rotation is influenced by several processes operating at different timescales. Although the planet’s rotation has slowed over very long periods, it has also experienced shorter intervals of faster rotation.
That recent acceleration raised the possibility that timekeepers might eventually have to remove a second from UTC to maintain the existing alignment.
A negative leap second would be unprecedented. Software that has been designed around adding a second may not handle a second being removed in the same way. The risk of inconsistent timestamps and failures across interconnected systems helped accelerate the push for reform.
By widening the permitted difference between UTC and Earth’s rotational time, the new approach removes the need to manage these one-second corrections routinely.
What about Russia’s opposition?
Russia previously raised concerns about moving away from leap seconds because its GLONASS satellite-navigation system relies on precise timing.
Changing the international standard requires countries and infrastructure operators to coordinate their systems and ensure compatibility. Russia’s concerns reflected the technical costs of that transition, particularly for satellite navigation and other critical applications.
The wider international decision seeks to provide a common standard while giving affected systems time to adapt to the new arrangement.
A new chapter in the history of time
The leap second has connected modern atomic timekeeping with Earth’s natural rotation for more than five decades. Its phaseout reflects the growing importance of uninterrupted, highly synchronised time in a world dependent on digital infrastructure.
The new system will preserve the ability to measure Earth’s rotation without requiring clocks to be adjusted every time the two measures drift apart by a small amount.
For everyday users, little will change on May 20, 2027. For scientists, engineers and operators of critical infrastructure, however, the decision represents a significant change in how the world’s clocks will remain synchronised over the coming centuries.