The end of the leap second.. Will world time change?

From the sun to atomic clocks

Before the advent of modern timekeeping methods, cities set their clocks according to the movement of the sun, which led to local time differences from one place to another. This difference had limited impact when journeys took days, but it became a problem with the expansion of railways in the nineteenth century.

In Britain, railway companies began adopting a unified time for their stations in the 1840s, based on Greenwich Mean Time. The telegraph then contributed to transmitting time signals, before the International Meridian Conference, held in Washington in 1884, established Greenwich's position as the primary reference for measuring longitudes.

With the development of mechanical clocks, the accuracy of time measurement improved, but the biggest shift came with the emergence of atomic clocks, which rely on the physical properties of atoms rather than the movement of gears or the rotation of the Earth.

In 1967, scientists adopted a definition of the second based on the cesium-133 atom, such that the second is equal to the duration of 9,192,631,770 cycles of radiation corresponding to a specific atomic transition.

Why did the leap second appear?

Despite the precision of atomic clocks, the Earth's rotation is affected by multiple factors, including the movement of the atmosphere and oceans, changes within the planet, and the tidal effects resulting from the gravity of the moon and the sun.

These factors lead to slight changes in the length of the day, while atomic clocks continue to measure time at a highly regular rate.

To address this discrepancy, the leap second was introduced in 1972, with the aim of keeping the difference between Coordinated Universal Time (UTC) and astronomical time related to the Earth's rotation within less than 0.9 seconds.

When necessary, a second is added to universal time, creating an exceptional moment of 23:59:60 before transitioning to the next day. Since the adoption of this system, 27 leap seconds have been added, the last of which was at the end of 2016.

However, this solution, which seemed appropriate at first, became a source of technical challenges with the expansion of digital systems that require precise and continuous synchronization.

A decision to end second adjustments

In October 2026, the General Conference on Weights and Measures made a decision to change the mechanism for handling the leap second, with implementation to begin on May 20, 2027.

The decision allows Coordinated Universal Time to continue without adding or removing leap seconds, while permitting the difference between atomic time and astronomical time to accumulate to a new maximum limit of 3600 seconds, or one hour.

This does not mean that the world will add or remove a full hour in 2027; rather, the difference will increase gradually over time, and it may take centuries to reach a full hour.

This trend was adopted to reduce the risks of disruptions that could result from inserting an extra second or deleting a second from computer systems, communications, power grids, and digital services. These systems do not all handle the leap second in the same way, and different processing could lead to errors in the ordering of events or device synchronization.

Concerns are growing about the potential future need for a negative leap second, i.e., deleting a second instead of adding one, a step that has never been implemented in the Coordinated Universal Time system.

Will satellites and navigation be affected?

It might seem that abandoning the leap second will affect the accuracy of positioning, but not all space navigation systems rely on Coordinated Universal Time in the same way.

The Global Positioning System (GPS), for example, uses a special atomic time to which leap seconds are not added, and it currently differs from Coordinated Universal Time by 18 seconds.

Navigation accuracy depends on calculating the signal travel time between satellites and receivers, in addition to knowing the Earth's position and orientation precisely. Therefore, stopping leap second adjustments does not mean ignoring the Earth's rotation, as scientific institutions will continue to monitor its changes and provide the data necessary for astronomical and navigational calculations.

In other words, the decision aims to decouple the regularity of atomic time measurement from the natural changes in the planet's rotation, not to neglect those changes.

Who determines world time?

No single country determines world time; rather, the system is based on international scientific cooperation in which measurement institutes and specialized organizations participate.

The International Bureau of Weights and Measures plays a pivotal role in calculating Coordinated Universal Time based on data from atomic clocks distributed in laboratories around the world, while the General Conference on Weights and Measures is responsible for adopting decisions related to international measurement standards.

This system reflects the shift of time measurement from relying on local astronomical observations to a unified global system based on precise physics and cooperation between nations.

Has the Greenwich era ended?

The decision does not mean abolishing Greenwich Mean Time or changing the reference meridian that bears its name. There is a difference between Greenwich Mean Time, which has historically been linked to the apparent movement of the sun, and Coordinated Universal Time, which is based on atomic clocks.

In daily civil use, the readings of the two times coincide in many contexts, but they differ in their scientific basis and precise definition.

The decision will also not change the time zones of countries, as the locally adopted differences will remain in place unless the competent authorities decide to modify them.

At the same time, scientists continue to develop more accurate optical atomic clocks, which may pave the way for redefining the second in the future, in addition to research aimed at creating a time reference for the moon, where gravity and motion affect the rate at which time passes.

Thus, the end of the leap second does not represent the end of world time, but rather a shift in how it is organized; after more than half a century of trying to keep atomic clocks in sync with the Earth's rotation, the world is moving toward prioritizing the continuity of digital time, while continuing to monitor the planet's movement and understand its changes.