Delegates meeting near Versailles are voting on whether to permanently abandon the leap second, replacing irregular atomic adjustments with a potential future leap hour as modern computer systems struggle with time running backward.
For decades, timekeepers have quietly slipped extra moments into our days. On 27 separate occasions since their introduction in 1972—most recently at the end of 2016—clocks have momentarily reversed course to keep ultra-precise atomic measurements aligned with Earth’s irregular and gradually slowing rotation.
That era may be drawing to a close.
Why Earth’s Rotation Keeps Shifting Beneath Our Clocks
By traditional definition, a second is an 86,400th of a day. Yet the planet refuses to spin at a uniform pace. Earth’s rotation is gradually slowing, largely because of the Moon’s gravitational influence and tides.
Shorter-term variations can also cause the planet to spin faster or slower. Since around 2020, Earth has been rotating faster. Volcanic activity, changes in the distribution of mass within the planet, melting glaciers, ocean currents and atmospheric conditions are among the other variables that influence its spin speed.
These measurements help determine when the planet’s rotation has drifted sufficiently from Coordinated Universal Time (UTC) to warrant a leap second. UTC is tracked by atomic clocks, which standardise the world’s measurements using the behaviour of caesium atoms, though newer nuclear clocks may soon surpass them. These measurements are maintained by institutions such as Australia’s National Measurement Institute.
How Irregular Time Adjustments Crash Modern Software
When those assumptions break, infrastructure fails.
Unplanned time reversals have previously contributed to crashes affecting internet technology provider Cloudflare and social media platform Reddit.
Yet the ongoing debates have gained fresh momentum due to Earth’s recent speed-up, which opens up the prospect of a negative leap second—a mechanism that would subtract time instead of adding it. Researchers caution that implementing this kind of correction could trigger unprecedented software glitches, as digital networks have no prior experience dealing with it.
The Proposal for a Leap Hour and the Century-Long Drift
If the delegates approve the current proposal in Versailles, the international time standard will be decoupled from the planet’s immediate spin. If approved, the change would allow Coordinated Universal Time to drift further from Earth’s rotation, potentially replacing leap seconds with a future “leap hour” to bring the two back into alignment.
Taking into account the ongoing long-term pattern where Earth’s rotational pace drops by under a second annually, accumulating a full hour of variance would require upwards of a thousand years.
While some people have a philosophical objection to losing leap seconds because it disconnects how we count time from our actual experience of it, supporters argue that precise timekeeping and monitoring Earth’s rotation can continue without frequent adjustments, freeing staff from checking time measurement problems.