A cross-disciplinary gathering of scientists, diplomats, and timekeeping authorities reached a collective decision on Thursday, 15 October, to phase out the practice of adding leap seconds to the global clock. Delegates convened under the auspices of the International Bureau of Weights and Measures in France brought a resolution to the table that had been under deliberation for many years.
The leap second — a periodic one-second adjustment that has kept atomic clocks aligned with Earth's actual rotation — is set to be phased out in favour of a leap hour, a far less frequent correction that timekeeping authorities say will reduce strain on the digital systems and national infrastructure that rely on precise time signals, while maintaining the underlying connection between coordinated global time and the planet's axial spin. A principal scientist at the UK's National Physical Laboratory noted that the change follows roughly three decades of deliberation within the international timekeeping community, and that humanity's oldest measure of time — the turning of the Earth — will remain the foundation from which the global time system is anchored.
Timekeeping worldwide runs on Coordinated Universal Time (UTC), a precision-based framework that atomic clocks have underpinned for over seven decades. Because the planet's rotational pace shifts under the influence of tidal forces, glacial melt, and seismic activity, a gap can open over time between astronomical timekeeping and the extraordinarily precise readings produced by atomic clocks. Periodically, timekeepers insert an extra second into UTC so that the clock-based standard stays aligned with how fast the planet actually spins.
Since 1972, the practice of inserting an occasional extra second into the official calendar has served as the standard mechanism for keeping atomic clocks aligned with the planet's actual rotation. Where a leap second lands on the calendar follows a fixed hierarchy: the final minute of December or June serves as the standard target, with March or September held in reserve for exceptional cases, and in every instance the adjustment takes effect at the stroke of midnight UTC. Roughly half a year before any adjustment would take effect, the Earth Orientation Center — operating under the International Earth Rotation and Reference Systems Service (IERS) — holds authority over whether an extra second gets added to the calendar.
Operational demands placed on technology companies, financial markets, and other time-sensitive industries have nonetheless exposed practical drawbacks in the leap second approach, even as it continues to serve its core synchronisation function. When a leap second was inserted into the global timekeeping system in 2012, the networking platforms LinkedIn and Reddit went offline, and the online ticketing infrastructure operated by Australian carrier Qantas stopped functioning. In a July 2022 communication, Meta's engineering team characterised leap seconds as a recurring operational burden for those managing hardware infrastructure, a challenge that has intensified alongside the faster rotational rate Earth has maintained over the preceding ten years.
Researchers in the field have pointed out that the accelerating spin of the Earth would have obligated official timekeepers to insert an unprecedented negative leap second into the global clock system. That adjustment, introduced in 2016, brings timekeeping into alignment with the planet's actual rotational pace.
Twenty-seven of these one-second corrections have been inserted into the global timescale to date, and the agreement reached in Paris leaves none scheduled to follow.
