The International Bureau of Weights and Measurements (BIPM) has officially ended the decades-long debate over leap seconds. The metrology body announced on Thursday that it will stop issuing these extra seconds to adjust Coordinated Universal Time (UTC) starting in May 2027. This decision resolves a technical dispute that has plagued the scientific community for thirty years and removes a recurring source of instability for global technology infrastructure.
Leap seconds were introduced in 1972 to bridge the growing gap between atomic time and the Earth’s rotation. Atomic clocks measure time with extreme precision, but the Earth’s spin is slowing down due to tidal friction from the moon and shifts in the planet’s molten core. To keep UTC aligned with astronomical time, known as Universal Time (UT1), scientists occasionally add a second to the clock. Since 1972, 32 leap seconds have been added, with the most recent one occurring in 2016.
Preventing a Negative Leap Second
The primary driver for abolishing the leap second is the rising risk of a negative leap second. In recent years, the Earth has accelerated its rotation, partly due to climate change and the shifting of gravity caused by melting ice. A 2024 study published in Nature suggested that a negative leap second could be required as early as 2029.
While positive leap seconds have caused glitches in computer systems for decades, a negative leap second has never been tested. Patrizia Tavella, director of the BIPM time department, stated that the probability of a negative leap second convinced the organization to set the application date for the abolition in 2027. This urgency was heightened after tests by an industry standards group revealed several failures when simulating a negative leap.
“The probability of a negative leap second convinced us to set the application date in 2027,” Tavella wrote in an email to Wired. She added that the group is waiting until May 20, 2027, to implement the change. This date marks the anniversary of the 1875 signing of the Metre Convention, which created the BIPM, as well as Metrology Day.
Impact on Global Technology
The abolition of the leap second simplifies operations for tech companies and financial institutions that rely on UTC. Systems that track time to the nanosecond often struggle with the insertion of an extra second. Companies like Google and Meta have used a “smear” method to spread the leap second over a day, but other systems have failed. In 2012, Qantas experienced booking system failures due to a leap second.
Judah Levine, a fellow at the US National Institute of Standards and Technology, noted that there are continuing problems with positive leap seconds even after 50 years. He emphasized that the concern over a negative leap second was that its effects had never been tested. “Life will have one less hassle,” Levine said in an email.
Tavella confirmed that the change will make life easier for system administrators. “The nice fact is that we have nothing to do,” she said. “Just continue to accumulate seconds and normal minutes of 60 seconds.”
Atomic time and astronomical time will continue to drift apart without leap seconds. To address this, the BIPM agreed to create a leap hour for the distant future. However, this adjustment is millennia away, as it would take thousands of years for the discrepancy to accumulate to one hour. Tavella stated that the BIPM does not speak of a leap hour yet. She noted that systems relying on UTC have nothing to do, while those relying on UT1 will need to use the information UT1-UTC, which will eventually exceed one second.
Source: Wired
