Leap seconds face judgment day as time lords prepare to vote

Proposal would let UTC drift an hour from Earth's rotation, sparing computer systems awkward adjustments for centuries

It's about time: the General Conference on Weights and Measures looks set to end leap seconds in an effort to reach horological harmony.

Since 1972, 27 leap seconds have been added to Coordinated Universal Time (UTC) to keep it in step with the astronomical time scale UT1, which is based on Earth's fluctuating rotation.

The proposal would increase the permitted difference between UTC and UT1 from 0.9 seconds to one hour, avoiding leap-second adjustments for centuries.

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Earth's rotation has also sped up in recent years, raising the possibility of a negative leap second – removing a second rather than adding one. That has never been done before.

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Experts have estimated a 30 percent probability that global systems will need a negative leap second by 2035 to keep the two time measures in step. That could pose problems for computer systems designed to handle added seconds but not removed ones.

Many national bodies support the proposal, which is expected to pass. If approved, the change would take effect on May 20, 2027.

Setnam Shemar, principal scientist at the UK's National Physical Laboratory, said the proposal would avoid the risks associated with leap seconds for centuries.

"Leap seconds can be difficult to implement reliably, and if not applied correctly, they can cause differences in time across networks, potentially leading to the failure of systems such as mobile phone networks or electricity grids. These risks could increase if a negative leap second had to be implemented for the first time.

"Previously, leap seconds were added to UTC to keep it within 0.9 seconds of UT1. From 20 May 2027, a difference of up to 1 hour will be allowed before intervention is needed, ensuring the continuity of UTC for several centuries or more.

"The relationship between UTC and UT1 is important for some applications, such as astronomy and celestial navigation. The decision would not sever that link: UT1 will continue to be monitored, and its difference from UTC will continue to be disseminated through established international services." ®