The International Bureau of Weights and Measures will eliminate the leap second on May 20, 2027, according to an announcement from the BIPM time department. The decision aims to prevent computing failures triggered by negative leap seconds, which have never been tested.
The Hidden Dangers of Negative Leap Seconds
For 50 years, positive leap seconds have been periodically injected into Coordinated Universal Time to compensate for the Earth’s decelerating rotation. The threat of a negative leap second—where a single second would be dropped entirely from the clock—presents an engineering hazard.
“There are continuing problems with positive leap seconds even after 50 years, and the concern was that the negative leap second would be much more serious because its effects had never been tested,” Levine explains. Patrizia Tavella, BIPM time department director, confirmed those concerns in an email to WIRED, noting that the probability of a negative leap second convinced them to set the application date in 2027. Tests conducted by an industry standards group showed several failures with a negative leap.
Choosing Metrology Day for the Transition
The group is waiting until May 20, 2027, to implement it because it is the anniversary of the 1875 signing of the Metre Convention, creating BIPM, as well as Metrology Day.
“The nice fact is that we have nothing to do!” Tavella states, explaining that systems will simply continue to accumulate seconds and normal minutes of 60 seconds. Levine concurs that “life will have one less hassle.”
Addressing the Drift Between Atomic and Astronomical Time
Abolishing the leap second does not mean atomic time and astronomical time won’t continue to drift apart. The BIPM also agreed to create a leap hour. Because seconds accumulate into hours at a glacial pace, a leap hour is millennia away.
“We do not speak of leap hour yet,” Tavella notes. If systems rely on UTC, they have nothing to do. If they rely on UT1, they have to be able to use the information UT1-UTC which, in the future, will be larger than 1 second.
