Delegates from 69 countries meeting at the International Bureau of Weights and Measures in France voted on Thursday to abandon the leap second and adopt a so-called leap hour as the mechanism for keeping clock time aligned with the Earth’s rotation.
For decades, scientists and engineers have debated how best to reconcile coordinated universal time (UTC), kept by precise atomic clocks, with mean solar time, which is governed by the Earth's rotation. Small, periodic "leap seconds" have been added or subtracted as needed to keep the two within a second of each other. Those adjustments, however, have proved contentious — particularly for computer systems and networks that assume a continuous, monotonic time scale.
The delegates agreed that the leap second system had become untenable. The move to a leap hour preserves the long-term link between civil time and the position of the Sun in the sky while removing the need for frequent, unpredictable one-second insertions. According to the conference, the first use of a leap hour is not expected for thousands of years, meaning the change will have no immediate practical effect on daily life.
The International Bureau of Weights and Measures, which maintains global standards for time, weight and distance, said the decision follows decades of technical discussions and concern about how irregular small adjustments affected telecommunications, navigation and computer infrastructure. Implementing the new regime will require legal adjustments to definitions of civil time and coordinated work by standards bodies and industries that depend on precise timekeeping.
Supporters of keeping civil time closely tied to Earth’s rotation argued that a long-awaited, infrequent leap hour strikes a balance between astronomical tradition and the operational demands of modern technology. Critics have warned that any decoupling between atomic and solar time creates legal and social questions about how societies reckon days in the very long term, but the delegates noted those issues can be managed with planning given the very long timeline before a leap hour becomes necessary.
For now, clocks and networks will continue to run on atomic time without the frequent leap-second interruptions that have caused complications in some systems, while the rare leap hour provides a mechanism to correct the discrepancy when it eventually accumulates to a substantial level.