World Timekeeping Body Scraps the Leap Second; Last One Was in 2016

The International Bureau of Weights and Measures (BIPM, from its French name) said on Thursday that from May 2027 it will stop adding leap seconds to Coordinated Universal Time (UTC), the global time standard based on atomic clocks. Thirty-two such seconds have been added since 1972. The decision ends a debate among metrologists that has lasted three decades and avoids the prospect of the first-ever negative leap second, in which a second would be removed.

Why seconds were added

Leap seconds were introduced in 1972 because time kept by atomic clocks began to drift from time based on the Earth's rotation. That rotation is slowing, largely because of tidal friction linked to the moon, with the planet's molten core, oceans and atmosphere also playing a part. The extra seconds kept UTC aligned with astronomical time, known as UT1.

BIPM decides when one is needed by averaging hundreds of atomic clocks worldwide and comparing the result with astronomical time. The most recent leap second was added in 2016.

Trouble for technology

Many databases and APIs rely on UTC, so an extra second caused disruption. Companies such as Google and Meta spread it out in tiny increments over about a day, according to Judah Levine, a fellow at the US National Institute of Standards and Technology. Qantas, the Australian airline, had its booking system disrupted by a leap second in 2012.

The Earth has recently been spinning faster, partly because of climate change: melting ice has shifted the planet's mass distribution. A 2024 study in Nature found a negative leap second might be needed as early as 2029. Levine says its effects have never been tested. Patrizia Tavella, director of BIPM's time department, says tests by an industry standards group showed several failures with a negative leap second.

Why May 20, 2027

Tavella says the date was chosen because May 20 is the anniversary of the 1875 signing of the Metre Convention, which created BIPM, and also Metrology Day. Systems that rely on UTC will have nothing to do.

The leap hour

Atomic and astronomical time will still drift apart, so BIPM also agreed to create a leap hour. It is millennia away. Systems that use UT1 will in future need to handle a UT1–UTC difference larger than one second.