A vote is being held on whether to abandon the leap second, which is used to synchronize atomic time with astronomical time
Since the 1970s, so-called leap seconds have been periodically added to clocks—a measure designed to keep traditional astronomical time aligned with high-precision atomic time. However, adding an extra second can disrupt global technological infrastructure, leading to website outages and network failures. In 2022, a decision was made to abandon this practice starting in 2035. This week, during the 28th General Conference on Weights and Measures—which begins today in Versailles, France—a vote will be held on a proposal by timekeeping experts to stop adding leap seconds as early as 2027.
Maldonado - Intercalar
Dolores del Campo Maldonado
Director of the Mechanical Magnitudes and Engineering Division at the Spanish Metrology Center (CEM)
The 28th General Conference on Weights and Measures is taking place this week. It is the plenary body comprising all member countries of the Metre Convention—the second-oldest diplomatic treaty still in force—which makes all decisions regarding the International System of Units and meets every four years.
I will be attending as a member of the International Committee for Weights and Measures (the executive committee of the CGPM) and as head of the Spanish delegation.
Essentially, the decision to be made is to stop applying corrections to Coordinated Universal Time (UTC)—a time scale defined by atomic clocks—until the discrepancy between it and the time dictated by the Earth's rotation exceeds one hour. This means we will go several centuries without making such corrections.
Spain’s position is entirely positive. This issue has been under debate for over twenty-five years. During this period, users have developed and implemented alternative approaches to managing leap seconds; this has introduced unnecessary fragmentation and created risks regarding the resilience, reliability, and integrity of time synchronization within national critical infrastructures.
In Spain, the use of GNSS [Global Navigation Satellite System] time as a reference—particularly in the private sector—combined with the *de facto* coexistence of different GNSS time scales, poses growing risks stemming from the multi-second differences between those scales and UTC. These discrepancies can lead to errors, failures, or operational inefficiencies in telecommunications networks and other systems relying on GNSS-based time synchronization. Furthermore, they undermine the goal of maintaining UTC as the single, universally recognized international time reference. More broadly, these discrepancies affect Internet-based time distribution equipment—particularly systems that rely on GNSS-derived time or need to maintain coherence between different time references—given that most time servers in use depend on GNSS.
Spanish experts, particularly those from the Royal Institute and Observatory of the Navy (Spain’s designated institute for time and frequency), have actively participated in and contributed to the discussions, conferences, and user forums organized by the Consultative Committee for Time and Frequency (CCTF). As part of these activities, the CCTF has presented ongoing studies and analyses regarding the potential introduction of a negative leap second, based on a stochastic model developed jointly with specialists in Earth rotation.
In conclusion, we fully support the proposal submitted to the 28th meeting of the CGPM (2026) to implement a continuous UTC starting on May 20, 2027. We consider this measure necessary and timely, given the significant risks that could arise in the coming years from the first-ever introduction of a negative leap second, in light of the observed acceleration of Earth's rotation. Furthermore, establishing a one-hour tolerance would minimize the need for future UTC adjustments while ensuring its continuity and stability for centuries to come.
There is also another very interesting resolution that we hope will be approved: the definition of an international lunar reference time scale and its traceability to UTC. Spain fully supports this initiative as well.
