World’s Timekeepers Vote to Retire the Leap Second Starting in 2027

The leap second, the occasional extra tick added to the world’s clocks to keep them in step with the planet’s wobbly rotation, is on its way out. Delegates to the General Conference on Weights and Measures voted on Thursday in Versailles, France, to make Coordinated Universal Time continuous beginning May 20, 2027.

Under the resolution, the gap allowed between atomic timekeeping and the time kept by Earth’s rotation, known as UT1, will widen from less than one second to as much as an hour, or 3,600 seconds. The conference said that limit should keep official time free of interruptions for several centuries. Instead of frequent one-second corrections, a much rarer adjustment, sometimes described as a leap hour, could be made if the two scales ever drift that far apart; according to Scientific American, scientists expect that will not be necessary for more than 1,000 years.

Coordinated Universal Time, the reference used by everything from financial markets to satellite navigation, was established in 1960. Because Earth’s spin was slowing, the first leap second was added in 1972, and 27 have been inserted since. Each one has been a headache for computer systems, which do not naturally handle a minute with 61 seconds.

A newer worry pushed the issue to a decision. Earth’s rotation began speeding up in the mid-2010s, Scientific American reported, raising the prospect of the first-ever negative leap second, in which a second would be removed rather than added. Engineers have warned that such a step, never tested at scale, could act like a small version of the Y2K computer scare. A workshop held last year estimated about a 30 percent chance that a negative leap second would be needed by 2035. Nick Stamatakos, who leads Earth orientation work at the United States Naval Observatory, said avoiding negative leap seconds was a major reason to open the window between the two time scales, according to Scientific American.

The resolution says transportation, energy, telecommunications and space systems need a time scale without future adjustments, and it warns that inconsistent workarounds and the spread of connected devices and smart power grids have added risk. It also cautions that some users might abandon official time for satellite navigation time if the change were delayed. Radio time signals will have a transition period through 2035, and no later than 2040 where equipment cannot be replaced sooner.

Noël Dimarcq, a senior scientist at France’s National Center for Scientific Research who leads the conference’s advisory committee on time and frequency, said that after 30 years of debate the timekeeping community was ready for the change, according to Scientific American. In a statement to the conference, he said the new framework was expected to keep UTC continuous for several millennia.

The trade-off is philosophical as much as technical. Civil time will slowly drift away from the position of the sun, though by an amount no one will notice for many generations. The same conference also asked for a proposal on redefining the second itself, based on more precise optical clocks, to be presented in 2030, and set out steps toward a reference time scale for the moon.