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The Leap Second Is No More
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- DeepHorus
The leap second is officially dead. The International Bureau of Weights and Measures (BIPM) announced Thursday that it will stop issuing leap seconds in May 2027. The decision ends a three-decade metrological debate over reconciling atomic time with Earth's rotation.
Since 1972, 32 leap seconds have been added to Coordinated Universal Time (UTC). They compensated for the gap between atomic clocks and Universal Time (UT1), based on Earth's rotation. That rotation is slowing, largely because of tidal friction from the moon, while Earth's molten core, oceans, and atmosphere also affect spin rate.
Leap seconds are not scheduled like leap years. BIPM averages hundreds of atomic clocks worldwide and compares that time with astronomical time to decide when one is needed. The most recent was added in 2016. Although one second seems trivial, UTC underpins countless APIs, databases, and networks.
Adding leap seconds has disrupted technology. Google and Meta use a "smear" method, spreading the extra second over a day in tiny increments, said Judah Levine of the US National Institute of Standards and Technology. Qantas saw its booking system broken by a leap second in 2012.
The risk of a negative leap second made abolition urgent. A 2024 Nature study found melting ice and climate change shifted Earth's gravity and sped up rotation, possibly requiring a negative leap second as soon as 2029. BIPM time director Patrizia Tavella said the probability convinced the group to set the 2027 date. Industry tests "showed several failures with a negative leap," she told WIRED.
BIPM chose May 20, 2027, the anniversary of the 1875 Metre Convention, which created the bureau, and Metrology Day. Tavella said the change means "we have nothing to do" but continue accumulating seconds. The world's clocks will now run without an obscure adjustment that once threatened global systems.
