The world’s timekeepers are meeting on Thursday near Paris to decide whether to scrap the leap second. The change could separate the clocks people use from the Earth’s rotation and realign global timescales.
Global clocks are built on the assumption that the Earth completes one rotation on its axis in exactly 24 hours. In practice, the planet wobbles, so the time it takes to rotate is not perfectly exact, and its rotation is gradually slowing.
How leap seconds keep clocks aligned with the Earth
Since 1972, timekeepers have added a leap second every few years to keep clocks in step with the planet’s rotation. The most recent one was added on December 31, 2016, and no further leap seconds have been added since. In total, 27 have been added so far.
Clocks follow Coordinated Universal Time (UTC), a global standard that has rested on high-precision atomic clocks for more than seven decades. Atomic clocks measure time by tracking the vibration of atoms. The Earth’s rotation, however, runs independently of those clocks, which is why the two drift apart.
According to the National Institute of Standards and Technology (NIST), a nonregulatory federal agency within the United States Department of Commerce, leap seconds fall at the end of December or the end of June. The agency points to the gap between the precision of the atomic second and the Earth’s rotation as the reason leap seconds are needed. It said the gradual slowing of the planet’s rotation is steadily lengthening the solar second relative to the atomic second.
Technology and financial disruptions linked to leap seconds
Although leap seconds bring UTC back into line with the Earth, they have repeatedly caused problems for companies that depend on precise timing. Financial markets and data centres rely on timing accurate to milliseconds, and pressure to abolish the leap second has grown because of the complications it creates for them.
Several past incidents were cited in the report. In 2012, LinkedIn and Reddit crashed after a leap second was added. The same year, ticket bookings for Qantas, the Australian airline, collapsed after the adjustment. Three years later, the Australian Securities and Investments Commission issued a warning about leap seconds.
The problems have continued to draw criticism from the industry. In July 2022, engineers at Meta said: “Every leap second is a major source of pain for people who manage hardware infrastructures.”
Versailles meeting weighs three proposals on timekeeping
The International Bureau of Weights and Measures, known by its French acronym BIPM, voted in 2022 to scrap the leap second by 2035. Russia has pushed for the date to be moved back to 2040. Russia builds leap seconds directly into its GLONASS satellite-navigation system, so a delay would give it more time to update its satellite systems.
Despite Moscow’s concerns, the 69 member states of the BIPM are gathering on Thursday in Versailles, outside Paris, to debate and vote on a resolution to scrap the leap second by 2027. According to the BIPM, timekeeping authorities from around the world will consider three proposals:
- Whether atomic clocks remain the most accurate way to define a second, given that optical frequencies are now more precise.
- Dropping the leap second altogether, which the BIPM described as the most significant of the proposals.
- Synchronising time measurements on Earth with a new time standard that NASA is developing for the moon.
If the proposal to change how time is measured is accepted, the second could be redefined in terms of optical frequencies rather than atomic vibrations. Proposals that are approved are expected to take effect from May 20, 2027, which is World Metrology Day.
What scrapping the leap second would mean for clocks
If the leap second is abolished, clocks will no longer be tied to the Earth’s rotation. The NIST said that adjustments would then have to be made to time stamps and to legacy equipment and software that synchronise to UTC for astronomical observations. It also noted that some proposals have called for larger corrections, such as leap hours, which would happen far less often than leap seconds.
Scientists said human routines would not be affected by abandoning leap seconds, and that software glitches, data server crashes and network failures in financial markets would be eliminated. The effects would instead appear over centuries. By 2100, clocks would be running 54 seconds ahead of the Earth.
Over a longer period, the gap would grow to several hours within about two thousand years. Sunrise and sunset times would then drift out of sync with people’s natural rhythms today.
