Atomic Time Faces Historic Change to Avoid Negative Leap Second

International time measurement organizations are preparing to vote on a reform that would effectively end leap seconds and allow UTC to drift up to one hour from the Earth's rotational time.

• The General Conference on Weights and Measures plans to vote in October on a resolution to make UTC continuous starting May 20, 2027.
• The Earth's faster rotation has increased the risk of needing a negative leap second, an operation for which many systems are unprepared.
• Experts estimate that the one-hour difference could take at least a thousand years to achieve if the proposal is approved.

International time organizations are preparing to decide whether the world should effectively abandon a tool that has kept atomic clocks synchronized with the Earth's irregular rotation for decades. The General Conference on Weights and Measures plans to vote in October on a draft resolution that would convert Coordinated Universal Time, known as UTC, into a continuous scale starting May 20, 2027.

The proposal would not immediately eliminate the relationship between official clocks and the planet's movement, but it would change the allowed margin between the two systems. Instead of requiring UTC to remain within 0.9 seconds of the Earth's rotational time, the reform would allow the difference to grow up to one hour, thus eliminating the need to periodically add or subtract seconds.

A Problem Born from Earth's Rotation

UTC is constructed from atomic clocks, whose stability allows for extremely precise time measurement and maintains a common reference for telecommunications, navigation, and computer systems. Universal time based on rotation, known as UT1, responds to a planet whose movement does not maintain a perfectly constant speed, due to physical variations that slightly alter the length of days.

Leap seconds arose to correct that difference and prevent civil time from drifting too far from solar noon. Since the introduction of the system in 1972, 27 leap seconds have been added to UTC, always under the logic of adding a fraction of time when the Earth's rotation lags behind atomic clocks.

The situation began to take on an unexpected nuance in recent years, as the Earth recorded a slight increase in its rotational speed. If this trend were to require the elimination of a second instead of adding one, officials would have to execute an unprecedented negative adjustment in the history of the system.

This possibility does not pose a threat to the daily lives of most people, who would likely not perceive the loss of a second. The risk is concentrated in digital infrastructures that rely on exact timestamps and whose programs, protocols, or servers were designed under the assumption that leap seconds are always added, as explained by Setnam Shemar, chief scientist at the UK's National Physical Laboratory.

The Fear of a Disappearing Second

Shemar warned that many digital architectures and network time servers do not properly account for the removal of a second. In his view, such an operation could cause severe disruptions in critical national infrastructures, including telecommunications, power grids, and satellite navigation systems, sectors where an unexpected timestamp can affect coordination among multiple devices.

The difficulty lies not only in a clock showing an incorrect time for an instant. Distributed systems can compare records, order transactions, coordinate signals, or validate events based on time sequences, so an absent second could generate different behaviors among servers that receive the adjustment unevenly.

The possibility of a negative leap second before 2035 has been highlighted in the debate about the future of UTC, although it does not imply that the adjustment will occur. The potential risk helps explain why a possibility that once seemed remote has begun to influence the international agenda.

The problem also affects positive leap seconds, although those adjustments already have precedents and known procedures. The combination of a practice that is difficult to implement, heterogeneous technological systems, and a possible negative correction has led experts to seek a solution that reduces the frequency of interventions rather than perfecting each individual adjustment.

The Proposal to Allow UTC to Drift

The draft that will be put to a vote proposes that UTC be continuous starting May 20, 2027, and that the difference with UT1 can gradually widen. If approved, the change would maintain a stable atomic reference for digital infrastructure, although it would allow civil time to drift further from the exact position of the Sun compared to the time used today.

According to Shemar, this decision would eliminate for many years the risks associated with new leap seconds. The international debate reflects the interest of various national bodies in ending a practice they perceive as dangerous or difficult to manage for their critical infrastructures.

The proposal reflects an increasingly visible priority in modern timekeeping: to protect the continuity of technological systems, even if that means accepting a greater separation between atomic time and the planet's rotation. For users, the difference would be imperceptible for generations, but for network operators, laboratories, and navigation systems, it would mean removing a periodic source of operational uncertainty.

The debate does not suggest that UTC will lose its function as an international reference or that each country can define its own time without coordination. The resolution seeks to maintain a common and predictable scale while changing the threshold at which bodies must intervene to correct the distance with rotational time.

A Slow Change with Technological Impact

The difference of one hour would not appear immediately nor constitute the expected outcome for the coming decades. Shemar calculated that UTC will likely not separate from rotational time by a full hour for at least a thousand years, a horizon so distant that the systems and needs of civilization could be completely different.

This timeframe explains why the proposal should not be interpreted as a warning that everyday clocks will soon be out of sync with the Sun. The immediate goal is to avoid a correction of a single second that could produce errors in connected infrastructures, while granting future generations the task of deciding how to manage a much larger difference.

The discussion has a technical dimension and an institutional one, as the decision will depend on a vote by national delegates from different countries. The pressure to abandon leap seconds mainly comes from accumulated concerns about their effects on national infrastructure.

The change also reveals a familiar tension for any global system: a rule created to maintain physical accuracy can become a source of vulnerability when applied to digital networks designed with different assumptions. In this case, timekeepers prefer to tolerate a slow and predictable deviation rather than risk an instantaneous modification that numerous systems may not be able to process.

The vote in October will therefore be the next decisive step in redefining the relationship between UTC and Earth's rotation. If the resolution receives support, leap seconds could be relegated to history long before the difference between the two clocks becomes a visible problem for society.