The End of the Leap Second: What Changes in Global Timekeeping
In the afternoon of October 15, 2026, delegates from 69 countries voted unanimously in Versailles, France, on a decision that fundamentally alters one of the cornerstones of time measurement: the permanent elimination of the leap second. The General Conference on Weights and Measures (CGPM), meeting in its 28th session, approved the resolution taking effect on May 20, 2027, replacing the atomic correction mechanism that had operated since 1972 with a system based on a "leap hour" — a change that in practical terms means atomic clocks and astronomical time can diverge by up to one hour before any further adjustment.
What is a leap second and why it existed
The leap second was introduced in 1972 as a synchronization mechanism between two fundamentally different timekeeping systems. On one side is International Atomic Time (TAI), based on the vibration of a caesium-133 atom — exactly 9,192,882,340 oscillations per second. On the other is astronomical time (UT1), determined by the Earth's rotation through space. The problem is that the Earth is not a precise clock: its axis wobbles, its rotation gradually slows due to the Moon's tidal forces, and geological events such as earthquakes can alter its spin rate by fractions of a millisecond.
To keep the divergence between atomic time (UTC) and astronomical time within 0.9 seconds, the scientific community decided to insert an extra second — the leap second — whenever necessary. Since 1972, 27 leap seconds have been added to global clocks. The most recent occurred on December 31, 2016. Each insertion makes one minute exceptionally last 61 seconds.
Why eliminate it?
The answer combines two factors: the unpredictability of Earth's rotation and global digital dependence on stable timekeeping.
The threat of a "negative leap second"
In 2020, the Earth completed 28 days spinning faster than predicted by atomic clocks. On June 29, 2022, the shortest day ever measured in the atomic clock era was recorded — Earth's rotation was 1.59 milliseconds faster than 86,400 atomic seconds. If this trend continues, experts warn of the possibility of the first negative leap second in history: instead of adding a second, one would be removed. Since no technological system has ever processed this operation, the outcome would be unpredictable.
> "Removing a second could result in a major failure in telecommunications networks, power grids, or satellite navigation systems," said Setnam Shemar, the UK's principal timekeeping scientist, to the Financial Times.
Impact on digital systems
The implications extend far beyond scientific curiosity. Financial markets, telecommunications networks, GPS systems, and electrical infrastructure depend on precise UTC synchronization. Leap seconds have already caused disruptions in operating systems and online services. Meta, formerly Facebook, was one of the loudest voices for elimination: in 2022, its engineers described the leap second as "a risky practice that does more harm than good."
The new rule: from seconds to hours
The solution adopted by the CGPM is radical in its simplicity. Instead of inserting irregular seconds, Coordinated Universal Time (UTC) can now diverge from astronomical time by up to 3,600 seconds — one full hour. This divergence will only be corrected when that threshold is reached, through a mechanism called the "leap hour." According to Noël Dimarcq, head of the BIPM's time and frequency committee, this should ensure an uninterrupted flow of time for "several centuries."
In practice, the effect will be similar to "living on daylight saving time permanently," according to Emily Akkermans, time curator at London's Royal Observatory. The change is essentially precautionary: experts believe it will take a long time before the divergence reaches the point of requiring correction, but they prefer to act before the first backward jump.
What this means for you
For everyday life, the news changes nothing immediately. Your phone's clock will continue functioning the same way. Train, flight, and meeting times remain unchanged. The difference is fundamentally technical: between the atomic clock and the actual position of the Sun in the sky, the discrepancy will grow slowly over the coming decades.
For professionals in astronomy, geodesy, and satellite-based applications, however, the change demands attention. These fields depend on precise knowledge of Earth's rotation. With the dissociation between UTC and astronomical time, they will need to calculate the growing divergence separately whenever necessary.
The future of the world's chronometer
The BIPM meeting in Versailles also opened the way for innovations going far beyond the elimination of the leap second. Proposals were solicited for a new definition of the second — needed because optical clocks are now significantly more accurate than the caesium clock that defines the current standard. Additionally, international agencies were invited to consider how to define a universal time standard for the Moon.
> "The changes will have a profound impact on future timekeeping both on Earth and other planets," Dimarcq said.
The era of the leap second comes to an end. What begins is a chapter in which atomic precision and Earth's rotation cease to be cosynchronized — and humanity decides, for the first time, that the consistency of digital time is worth more than the elegance of keeping atomic clocks aligned with the planet's rotation.
Sources: CNN, New Scientist, Euronews
✓ Independent sources cross-checked and verified before publishing
