Time is running out for the leap second

The writer is a science commentator

The leap second is living on borrowed time. This interloper, inserted into global clocks 27 times since 1972 and last added in 2016, allows two competing systems of time to run in parallel: astronomical time, long measured by our species according to the Earth’s slightly irregular rotation; and Coordinated Universal Time (UTC), the global time standard set by ultra-precise atomic clocks and used in infrastructure such as telecoms, power grids and data servers — plus phones and computers.

The correction irks infrastructure operators, who apply the fix in different ways. Some operators “smear” the extra second into their systems across many hours, rather than adding it in one go. Global timekeeping authorities had already voted to abolish the leap second by 2035.

But now there is a spanner — or spinner — in the works. Our planet has been spinning faster since about 2015, meaning that before the 2035 deadline, UTC might need to add a negative leap second. Deleting a leap second has never been tried and could, say scientists, create mayhem in the digital time-stamped continuum.

“Even positive leap seconds are hard to implement reliably,” explains Setnam Shemar, principal scientist at the UK’s National Physical Laboratory, who manages timing matters for the UK. “Many computer programmes assume all leap seconds are positive . . . [a negative leap second] could result in a major technology failure in telecommunications or electricity grid networks or satellite navigation systems.”

As the journal Nature reports, the horological crisis may condemn the leap second to an earlier grave. Next month, the world’s timekeepers will vote on whether to replace it with a “leap hour”. If the resolution passes, it could take a millennium or more for a leap hour to accumulate, in effect nullifying the need for any corrective action at all. It would be a practical win for tech overlords, digital gatekeepers and official clock-watchers but something of a cultural loss for our stargazing species, detaching human timekeeping from its ancient, astronomical roots.

Over millions of years, the Earth’s spin has been slowing very slightly due to the Moon’s frictional drag. The lag between astronomical time and UTC must not exceed 0.9 seconds; once that looks likely, a positive leap second is inserted into UTC to yank them back into alignment. But changes in the Earth’s core are now causing our planet to rotate faster, bringing a 30 per cent chance that a negative leap second will be needed by 2035.

The “leap hour” vote will be overseen in mid-October by the International Bureau for Weights and Measures, based near Paris and the guardian of UTC, as well as other measurement standards such as the kilogramme. Most of its 69 member states, Shemar said, want to get rid of leap seconds because of the risk posed to national infrastructure; moving to a leap hour “would remove the risks entirely for a very long time”.

Markus Kuhn, a computer scientist at Cambridge university, told me that a negative leap second should, in theory, cause less havoc than a positive one, because the clocks on computers will skip a second rather than add an extra one. “That should be less disruptive, as it doesn’t affect the relative ordering of time-stamped events. But it is also a new situation, that hasn’t been tested widely yet.” Kuhn has been against the idea of a leap hour since it was first floated more than two decades ago, because tweaks would happen so rarely: “The rarer an event is, the more disruptive it is.”

Shemar, a former astronomer, thinks “a leap hour would never be applied ever, since life would be so different on Earth thousands of years from now.” The immediate priority, he said, was to preserve safety-critical infrastructure, even if it means severing the cultural link between human timekeeping and the stars.

Timekeeping may, of course, look very different in the millennia to come. Our ancestors would surely marvel at the caesium atomic clocks that have superseded sundials. Even these might one day seem old-fashioned: newer atomic clocks that use ytterbium instead of caesium are said to keep time so faithfully that, had they started ticking at the birth of the universe nearly 14bn years ago, they would have lost less than a second.

Still, I shall mourn the probable passing of the leap second, whose lifetime nearly mirrors my own revolutions around the Sun. I always thrilled at hearing the extra pip on the radio, a humbling reminder that we live on an erratically twirling rock orbiting a fireball, as chained as our ancestors to the unceasing rhythm of night and day.