On October 13th to 15th local time, the 28th International Measurement Congress was held in Versailles, France near Paris. The International Bureau of Weights and Measures voted at a meeting on the 15th to pass a resolution on “Technical measures to ensure continuity of Coordinated Universal Time (UTC),” which means that the half-century use of leap seconds worldwide will be discontinued.
This measure will come into effect on May 20, 2027, to ensure long-term continuity of the coordinated world time over the coming centuries. The scientific journal Nature's website states that it is not yet clear how each member state of the International Bureau of Measurement will vote, but the resolution can be passed with the support of a majority of member states.
File photo: The headquarters of the International Bureau of Metrology located in the suburbs of Paris, France. Associated Press
Since the introduction of leap seconds in the Coordinated Time in 1972, 27 instances of positive or negative leap seconds have been implemented worldwide. The last adjustment was at the end of 2016. However, with the rapid development of information technology and the widespread adoption of digital infrastructure, the negative impacts caused by the leap second mechanism have been increasing. Recent increases in leap seconds have led to a series of problems, including website crashes and network service failures.
In recent years, scientists have also observed an abnormal acceleration in the Earth's rotation, which could lead to negative leap seconds in the future. This means that during coordinated timekeeping, a full second would need to be skipped. Since negative leap seconds have never occurred before, many experts are concerned that they could cause widespread system failures, resulting in serious consequences.
Due to concerns about risks, calls for canceling leap seconds are increasing. Many countries and companies hope to stop forcing changes to time.
The new standard adopted by the International Metrology Conference will reduce the maximum difference between Coordinated Time and Universal Time to 3600 seconds, or one hour. Scientists estimate that it will take more than 3000 years for such a difference to occur.
Regarding this significant adjustment involving the global timing system, there are different opinions in the scientific community. Some scientists are concerned that this means the currently widely used Coordinated Time will be completely “disconnected” from the Earth's rotation. However, other analysts point out that compared to when the leap second mechanism was introduced, today’s digital world has extremely high demands for timing accuracy. The problems caused by leap seconds have already outweighed their benefits.
To determine time, there are currently two main systems for measuring time in the world: Coordinated Universal Time (UTC), which is based on the Earth's rotation, and Atomic Time (TAI), which is based on atomic oscillation periods.
World time is a time standard determined by considering the time it takes for the Earth to complete one rotation as one day. It divides time scales based on the angle change of the Earth relative to the mean solar day. However, the Earth does not rotate at a constant speed. The Moon's gravitational force, internal changes on the Earth, ocean movements, and other natural factors can all affect the Earth's rotation, thereby impacting the measurement of world time.
With the development of atomic clock technology, the 13th International Conference on Metrology in 1967 decided to define the second based on the duration of 9192631770 periods corresponding to the transition between the two hyperfine levels of the ground state of cesium-133. This led to the creation of an atomic second that is more continuous and stable.
Due to the differences between these two time measurement systems, in order to reconcile the timing of atomic clocks with the Earth's rotational time, Coordinated Universal Time (UTC) was established starting from 1972. UTC is based on the atomic second, aiming to be as close to Universal Time as possible in terms of time. When there is a difference of more than 0.9 seconds between atomic time and Universal Time, UTC is adjusted by adding or subtracting 1 second—this is known as a leap second.
Since 1972, the Earth's rotation has been slowing down, and Coordinated Universal Time has accumulated 27 positive leap seconds.
China National Timekeeping Center of the Chinese Academy of Sciences, Beijing Time, CCTV News
Leap seconds are typically introduced at the last minute of the last day in June or December, by inserting a special moment at 23:59:60. However, compared to leap years that occur every four years, the occurrence of leap seconds follows no specific pattern. It is difficult for the scientific community to predict changes in Earth's rotation over the long term. The International Bureau of Weights and Measures only announces this change six months in advance.
A study funded by NASA has shown that climate change is also one of the factors affecting the Earth's rotation. The accelerated melting of the ice sheets in Greenland and Antarctica is leading to a rise in global sea levels, resulting in a slower rate of Earth's rotation in 2018 compared to 1900. The Guardian also pointed out that the current rate of Earth's rotation is much slower than it was 65 million years ago, when a full rotation took only about 23.5 hours.
However, things changed starting from 2020. Scientists say that although the long-term trend of the Earth's rotation has been a slowdown, in recent years there has been a trend of faster rotations. On June 29, 2022, the Earth's rotation reached its fastest record since 1969, with the time it takes to complete one rotation being 1.59 milliseconds less than the standard 24 hours.
This means that the world may need to subtract one second from the Universal Time in the near future, i.e., introducing what is called a “negative leap second”. The International Bureau of Weights and Measures previously warned that since negative leap seconds have never occurred before, operators of critical infrastructure “are basically unprepared” and could cause serious chaos in today’s highly digitalized world.
The institution noted that preparing for negative leap seconds would require significant funding for industries that rely on time synchronization. The cost could be comparable to the expenses incurred in dealing with the “Millennium Bug” issue at the turn of the century. The “Millennium Bug” refers to the flaw in early computer systems that failed to properly handle the transition from 1999 to 2000, a problem that required substantial global investment to address.
A German-based international organization for the study of Earth's rotation and reference systems estimated in a report published in 2025 that the likelihood of negative leap seconds in the coming years will increase significantly. The International Bureau of Measurement predicts that by 2035, the probability of negative leap seconds will reach 30%.
American National Standards Institute's Time and Frequency Division expert Judith Levin said, "When the leap second system was established in 1972, no one really expected negative leap seconds to occur. It was added to the standard for completeness purposes. Everyone thought that positive leap seconds were sufficient, but now, as the length of the day is decreasing, the possibility of negative leap seconds has greatly increased."
Levin pointed out that even though Coordinated Universal Time has been used worldwide for over 50 years, various problems still arise every time positive or negative leap seconds are implemented. Therefore, there is significant concern among various parties regarding the possibility of negative leap seconds occurring. “This has never been tried, nor has it ever been tested.”
The journal Nature points out that, in fact, positive and negative leap seconds already cause headaches for systems that involve time synchronization. This is particularly true for industries such as telecommunications and power grids, where any changes involving leap seconds can lead to various system failures.
After the addition of positive and leap seconds in 2012, synchronization issues with time caused some websites such as LinkedIn to crash. The booking system of Australia Airlines was also paralyzed for a while. The last time a leap second was added was in 2016, when Cloudflare, one of the world's leading internet infrastructure providers, failed to handle the leap second properly, resulting in temporary service interruptions or delays for customers.
