The International Bureau of Weights and Measures (BIPM), which maintains the International System of Units (SI), has voted to expedite the retirement of the leap second moving forward the date from 2035 to May 20, 2027, to address the mismatch between Coordinated Universal Time (UTC) and the Earth’s rotation.
BIPM Brings Forward Leap Second Retirement Date
When these two standards drift apart, standard-keepers traditionally insert a positive leap second into UTC. Coordinated Universal Time relies on atomic clocks as a precise baseline for civil time and digital networks, while UT1 is calculated from the Earth’s rotation, which fluctuates due to forces from the atmosphere, oceans, core, and the Moon.
Unlike leap years that follow a predictable four-year cycle, leap seconds cannot be pre-programmed by software developers because they depend entirely on unpredictable planetary movements. A positive leap second adds an extra second to the calendar year. Clocks went to 23:59:60 on Dec. 31, 2016, before switching to 00:00:00 on Jan. 1, 2017.
General Conference on Weights and Measures Decisions Target Negative Leap Second Risks
The standards organization warned that the change carries severe operational hazards. A negative adjustment remains entirely untested, whereas positive leap seconds have caused historical disruptions—such as a famous crash at Reddit following the 2012 addition of a leap second.
Major technology companies have previously developed custom, unstandardized workarounds to smooth over standard leap seconds, leaving smaller networks and Internet of Things (IoT) devices vulnerable to timing errors. The organization also estimated that upgrading global systems to safely handle the subtraction of a second would require investments similar to their preparations for the millennium bug.
CGPM Replaces Leap Seconds with Continuous UTC Framework
Rather than forcing sudden one-second interventions, the new model will permit a defasagem de até 3,600 segundos, equivalent to a one-hour tolerance gap between atomic time and planetary rotation. To eliminate these risks, the CGPM voted to scrap the traditional leap second mechanism entirely and replace it with a new framework known as Continuous UTC.
Proponents of the change argue that human societies already manage much larger time offsets routinely. International standards bodies intend for this wider tolerance to guarantee long-term stability for UTC, keeping it synchronized with International Atomic Time (TAI) for several centuries.
By permitting an accumulated offset of up to an hour under Continuous UTC, standard-keepers hope to protect national critical infrastructures—including transportation networks, power grids, telecommunications, and space exploration projects—from the cascading anomalies threatened by a negative second. The organization further noted that changes of an hour are regularly introduced when daylight-saving time is used.