VERSAILLES, France. For over half a century, civil timekeeping has walked a precarious tightrope. On one side stands International Atomic Time (TAI), dictated by the unyielding, ultra-precise vibration of cesium atoms. On the other stands astronomical solar time (UT1), driven by the slightly erratic, unpredictable spin of planet Earth.
To keep the two systems in harmony, metrologists created the leap second in 1972, periodically pausing or advancing world clocks by one second whenever the drift approached 0.9 seconds.
Now, that 54-year experiment is coming to an end.
Gathered at the 28th General Conference on Weights and Measures (CGPM) in Versailles, delegate scientists representing 69 member nations are voting on Draft Resolution C. The historic proposal seeks to officially discontinue the practice of inserting leap seconds into Coordinated Universal Time (UTC) as early as May 20, 2027, according to the BIPM resolution package.
The Fundamental Timekeeping Conflict ├── Atomic Time (TAI): Cesium beam clocks, perfectly uniform (1 second = 9,192,631,770 oscillations) ├── Astronomical Time (UT1): Earth's rotation, erratic due to tidal friction, core dynamics and ice melt └── Coordinated Universal Time (UTC): The civil compromise, held within 0.9s of UT1 using leap seconds
The Tech Nightmare | Why the World Wants to Ditch It
While inserting an extra second on New Year's Eve might sound harmless, it is a nightmare for digital infrastructure.
Because computer operating systems, telecommunications networks, financial stock exchanges, and satellite navigation systems (GPS, Galileo) rely on strictly linear, continuous time sequences, repeating a second, for example 23:59:60, frequently triggers catastrophic software crashes.
High-profile leap second events over the past two decades have repeatedly knocked major web platforms offline, disrupted airline reservation databases, and caused transaction errors across global stock markets. The NIST Time and Frequency Division has documented the recurring operational burden in its leap second background material.
Key Milestones in the Leap Second Timeline ├── 1972: UTC established with leap second correction rules ├── 1972-2016: A total of 27 positive leap seconds inserted into global clocks ├── 2022: BIPM votes at the 27th CGPM to abandon leap seconds by 2035 └── October 2026: CGPM votes on Draft Resolution C to accelerate the phaseout to May 20, 2027
The Threat of the Negative Leap Second
While major technology firms including Meta, Google, Microsoft, and Amazon have long lobbied to end leap seconds, the sudden urgency driving the October 2026 vote stems from a bizarre geophysical twist: Earth is speeding up.
Geophysical Mechanics of the Rotational Shift ├── Historical Trend: Earth's spin slowly decelerating from lunar tidal friction (about 1.5 ms per century) ├── Recent Anomaly: Core-mantle fluid dynamics and post-glacial rebound accelerating the spin └── Implication: Forces timekeepers to consider a negative leap second (skipping 23:59:59)
For decades, Earth's rotation was gradually slowing down, requiring scientists to add positive leap seconds. However, recent shifts in Earth's liquid iron core, combined with mass redistribution from melting polar ice caps, have unexpectedly accelerated planetary rotation.
That mass redistribution is the same physical process tracked in polar science. Researchers monitoring the plateau in Arctic sea ice melt seasons and the carbon stored in warming seabed permafrost are measuring changes that shift water from the poles toward the equator, subtly rearranging Earth's moment of inertia and nudging the length of a day.
According to technical studies presented to the International Bureau of Weights and Measures (BIPM), there is a 30 percent probability under current rules that a negative leap second would be required within the next few years.
Rather than repeating a second, clocks would be forced to skip 23:59:59 entirely, jumping directly from 23:59:58 to 00:00:00. Because a negative leap second has never been executed in human history, software engineers warned it could trigger unprecedented global network failures.
The Leap Second Transition Pipeline
[Current System (UTC)] --> Forced adjustment when the UT1-UTC gap exceeds 0.9s
|
v (Draft Resolution C vote)
[Continuous UTC System] --> No adjustments; tolerance relaxed to 3,600s (1 hour)
|
v
[Future Decisions] --> Later generations evaluate leap hour corrections
Enter Continuous UTC and the Leap Hour
If Draft Resolution C passes as expected, it will replace traditional UTC with a continuous UTC time scale starting May 20, 2027.
Under the proposed rules, metrologists will allow the gap between atomic time (UTC) and solar astronomical time (UT1) to drift freely. The draft resolution formally increases the maximum allowed difference from 0.9 seconds to 3,600 seconds, or one hour.
| Metrological Parameter | Current Standard (1972 Rules) | Proposed Continuous UTC (2027) |
|---|---|---|
| Max drift limit (UT1 minus UTC) | 0.9 seconds | 3,600 seconds (1 hour) |
| Adjustment frequency | Irregular, with 6-month advance notice | None for several centuries |
| Long-term consequence | Solar noon stays tightly aligned with 12:00 PM | Solar time slowly drifts out of sync over millennia |
| Estimated first correction | Repeated short-term fixes | Leap hour adjustment around A.D. 3500 to 4000 |
Scientists estimate that without leap second adjustments, it will take over 2,000 years for atomic time and Earth's position relative to the Sun to drift apart by a full 60 minutes.
When that milestone is eventually reached in the far distant future, society will simply implement a single, planned leap hour, shifting civil time in a manner identical to Daylight Saving Time, effectively kicking the timekeeping problem down the road for future millennia to handle.
Precision Measurement Has Its Own Costs
The trade-off is real but narrow. Physicists who need Earth orientation for its own sake, for radio interferometry, deep-space navigation, and geodesy, do not lose access to UT1. Observatories will keep publishing the difference; it simply stops being stitched into the clocks that billions of consumer devices read.
It is the same discipline that makes precision timekeeping possible at all. The cesium-definition work behind TAI underpins instruments as different as an atomic clock and the 5,160 optical sensors of the IceCube neutrino detector, which must timestamp faint light flashes across a cubic kilometer of Antarctic ice to nanosecond accuracy. More on how measurement standards and physical observation interact lives in our science coverage hub.
Frequently Asked Questions
The vote in Versailles settles a dispute that has run since 1972 by choosing engineering certainty over astronomical tidiness. For the first time since atomic time became the world's civil clock, no one will have to decide what happens at 23:59:60. The second, as a unit of official time, simply stops being negotiable.