Global vote to replace leap seconds with leap hour in timekeeping

Consensus Summary

Global timekeepers voted in October to replace the problematic leap second with a ‘leap hour,’ marking the end of a decades-long debate. Delegates from 69 countries gathered in Versailles (12–15 October) under the International Bureau of Weights and Measures (BIPM) to approve the change, driven by concerns that leap seconds—added 27 times since 1972—disrupted computer systems, including crashes at Reddit and LinkedIn in 2012. The decision was accelerated by Earth’s unexpected speedup since about 2020, which risked requiring a ‘negative leap second’ (time jumping forward) in the next decade, a scenario never tested. Under the new system, UTC can drift up to an hour from Earth’s rotation before a leap hour corrects it, though this correction may not be needed for more than a millennium.

The leap second was introduced in 1972 to align atomic clocks with Earth’s irregular rotation, which varies by a millisecond daily due to factors like the Moon’s gravitational pull, ocean currents, and glacial melt. Atomic clocks, defined by 9,192,631,770 caesium atom vibrations since 1967, provide the precision modern technology demands. However, leap seconds—added irregularly—caused technical chaos, as software assumed time only moves forward. The UK’s National Physical Laboratory (NPL) and Australia’s National Measurement Institute (NMI) have long maintained these clocks, with NMI staff historically working on New Year’s Day to address leap second issues. The leap hour proposal, while reducing immediate risks, raises philosophical questions about disconnecting timekeeping from Earth’s natural rhythms.

Key figures emphasized the leap hour’s practical benefits. Dr. Setnam Shemar of the NPL called the leap second ‘tiresome,’ arguing the new system protects global infrastructure like electricity grids and satellites from disruptions. Dr. Michael Wouters of Australia’s NMI noted that leap seconds had already severed the link between human timekeeping and Earth’s rotation, a concern shared by philosophers. Meanwhile, Russia opposed the change, citing potential impacts on its Glonass satellite navigation system. The IERS Earth Orientation Centre in Paris, which tracks Earth’s rotation, approved the vote but framed it as a distant solution, with Dr. Christian Bizouard calling it a ‘philosophical discussion’ deferred to future generations.

While both sources agree on the core decision, they differ in emphasis. The ABC highlights Earth’s millennial-scale rotation changes and the black hole observations used to measure its speed, while the Guardian focuses on the 1955 invention of the caesium clock and the 2012 leap second failures. The ABC also notes that leap seconds were often added on December 31, a detail omitted by the Guardian. Both agree the leap hour is a long-term fix, but the Guardian stresses the immediate risks of negative leap seconds, whereas the ABC frames the debate as balancing technological needs with philosophical objections to altering time’s human experience.

The unresolved question is whether the leap hour will ever be implemented. Both articles confirm it could take more than a millennium before the drift reaches an hour, rendering the change largely symbolic for current generations. The ABC suggests software automation in 100 years may eliminate leap second problems entirely, while the Guardian treats the decision as a precautionary measure against future unknown risks. With the vote now cast, the focus shifts to how global systems will adapt—and whether the leap hour will remain a theoretical solution or become a practical necessity in an era of increasingly precise technology.