On October 15, 2026, the General Conference on Weights and Measures (CGPM) adopted a resolution to operate Coordinated Universal Time (UTC) without leap-second adjustments starting May 20, 2027. The system has until now kept UTC within 0.9 seconds of time based on Earth's rotation; the tolerance will be widened to one hour. Behind the change is concern that a "negative leap second," which would remove a second and has never been carried out, could cause failures in communications networks and other systems. According to the announcement from the International Bureau of Weights and Measures (BIPM), the change allows UTC to advance continuously, and it does not remove the need to track Earth's rotation accurately. By separating the roles of the world's common clock and the observation of Earth's rotation, the aim is to make time-synchronization systems more stable.
From 0.9 seconds to one hour: what changes
At the 2022 General Conference, it was decided that the tolerance between UTC and UT1, the universal time based on Earth's rotation, would be widened by 2035. Resolution 5, adopted this time, sets the date of implementation as May 20, 2027 and formally fixes the new upper limit at 3,600 seconds, or one hour. What had been described as a plan to abolish leap seconds by 2035 has now become a decision with a specific implementation date and operating conditions.
Leap seconds are a mechanism that keeps the difference from UT1 within a set range by adding a second to UTC or, if necessary, removing one. According to records from the National Institute of Information and Communications Technology (NICT), 27 leap seconds have been implemented since the system began in 1972, and all were "positive leap seconds" that added a second. The last one took place at the end of December 31, 2016 in UTC, just before 9:00 a.m. on January 1, 2017 in Japan time. In a positive leap second, "23:59:60" is inserted after the usual 23:59:59 before the clock moves on to midnight of the following day. This compensates for the amount by which Earth's rotation falls behind time based on atomic clocks, by giving the clock an extra second.
Under the new system, such one-second adjustments will stop and UTC will advance continuously. However, clocks will not be moved forward or back by an hour on the effective date of May 20, 2027. The one-hour figure is an upper limit on the divergence that will accumulate between UTC and UT1 from now on, and the resolution expects UTC to be operable continuously for several centuries. When that limit might eventually be reached, or whether a large correction such as a "leap hour" would be implemented at that point, is not specified with a date or procedure in this resolution.
UTC, UT1 and TAI: two time differences that are easy to confuse
Key to understanding this change are the differences among UTC, UT1 and International Atomic Time (TAI). UTC and TAI tick seconds at the same rate, and UTC, which is produced by the BIPM, is time that differs from TAI by a whole number of seconds. UT1, on the other hand, is determined by observing Earth's rotation angle and reflects variations in the planet's rotational speed. In other words, the difference lies in whether time is kept stably by atomic clocks or based on how Earth is actually rotating.
What is being widened to one hour is the tolerance between UT1 and UTC, which is separate from the current 37-second difference between TAI and UTC. Sorting out this distinction makes it easier to see what the resolution changes and what it leaves unchanged. The comparison below is based on Resolution 4 of 2022, Resolution 5 of 2026, IERS Bulletin C 72, and Resolution 4 of 2026.
| Item compared | Current relationship | Change under the resolution |
|---|---|---|
| Absolute difference between UT1 and UTC | Upper tolerance of 0.9 seconds | Widened to 3,600 seconds from May 20, 2027 |
| Difference between TAI and UTC | TAI is currently 37 seconds ahead | No provision to eliminate the difference that has built up |
| Definition of the second | Based on a specific transition frequency of the cesium-133 atom | Definition unchanged by making UTC continuous |
The current 37-second gap between TAI and UTC includes the 10 seconds that existed when the system began in 1972, plus the 27 seconds added by the 27 positive leap seconds since. Bulletin C 72, issued by the International Earth Rotation and Reference Systems Service (IERS), confirms that UTC is 37 seconds behind TAI and that no leap second will be implemented at the end of December 2026. This 37 seconds is not the result of errors accumulating in atomic clocks; it is a difference established by convention between the two time scales. Widening the tolerance between UT1 and UTC to one hour therefore does not eliminate the existing difference between TAI and UTC.
In the first place, leap seconds were never meant to make up for insufficient accuracy in atomic clocks. According to an explainer from the U.S. National Institute of Standards and Technology (NIST), the length of the atomic second was historically fixed by comparison with "ephemeris time," which is based on Earth's orbit, and it inherits the property of being slightly shorter than the mean solar second. Added to this are variations in Earth's rotational speed, so the difference between time based on atomic clocks and time based on Earth's rotation gradually accumulates. What is being relaxed is the operating condition for keeping that difference small; it does not change the accuracy of atomic clocks or the length of the second.
Why abolish leap seconds without waiting until 2035
Behind bringing forward the abolition of leap seconds is the possibility that a "negative leap second" could become necessary because of variations in Earth's rotational speed. If Earth's rotation speeds up and UT1 continues to run ahead of UTC, a second would have to be removed from UTC to correct the difference. Under the handling of a negative leap second described by NIST, 23:59:58 would be followed by 0:00:00 of the next day, skipping 23:59:59. Such an operation is provided for in the rules, but it has never been applied to UTC anywhere in the world.
That said, it has not been established that Earth's rotation will continue to speed up in one direction. Over the long term Earth's rotation tends to slow, while over the short term it speeds up and slows down. Preparing for a negative leap second is not based on a forecast that rotation will keep accelerating at a steady rate; it is a judgment that variations could make one necessary.
Section 4 of the technical report compiled by the Consultative Committee for Time and Frequency (CCTF) describes an assessment made at an expert meeting held by the BIPM and IERS in 2025. The researchers divided variations in Earth's rotation into factors that are relatively predictable and irregular factors that are not well understood, and used a stochastic model for the latter. After confirming that three independent research groups had obtained similar results, they checked the agreed model against rotation observation data going back several centuries to verify its validity.
This assessment estimated the probability that a negative leap second would be needed in the roughly ten years after the expert meeting at about 30%, and the probability that a positive leap second would be needed at about 50%. Resolution 5 also puts the probability that a negative leap second will be needed by 2035 at 30%. However, this is neither an announcement that a negative leap second will be carried out in a particular year nor a statement that time-synchronization systems have a 30% chance of failing. The possibility that a positive leap second will be needed also remains, and there is uncertainty about how Earth's rotation will change. Even so, getting equipment and software around the world to handle a negative leap second, which has no track record, would require large-scale testing. Given such risks, it was judged that abolishing leap-second adjustment itself would better ensure the stability of systems that depend on time synchronization than waiting until a negative leap second is actually needed.
Why a one-second adjustment caused a DNS failure
The risk that leap seconds can cause system failures has already been shown by past cases. On January 1, 2017 (UTC), Cloudflare experienced a failure in which some DNS name resolutions failed in connection with handling a positive leap second. According to the company's incident report, the cause was that time moved backward, so calculating the time a DNS server took to respond produced a negative value.
The software at the time smoothed DNS server response times and used the result to choose which server to use. But when a negative value that had not been anticipated as a response time was passed in, an exception occurred in the server selection process. The problem is not only whether a clock can display the special time "23:59:60." The breakdown of a condition that programs take for granted, that a time obtained later is ahead of one obtained earlier, can also lead to failures.
In Cloudflare's case, some name resolution was affected for customers using CNAMEs that refer to other domain names. At the peak, about 0.2% of DNS queries were affected, and communications around the world did not stop. Even so, it is a case showing that even positive leap seconds, which had been carried out many times before, can cause unexpected failures in everyday software processing.
To avoid such abrupt changes in time, Google recommends a method called "leap smear." Google's technical documentation describes a method of adjusting the rate at which clocks advance little by little over the 24 hours from noon UTC to noon the next day. With this method there is no need to suddenly move the clock forward or back by a second, and after the adjustment period ends, the clock again matches UTC. However, during the adjustment, the time temporarily deviates from strict UTC.
Furthermore, the period over which smearing is carried out and how clocks are advanced are not uniform across all operators. If each company adopts its own method, time mismatches could instead arise between systems synchronized by different methods. Leap seconds, introduced to maintain a common world time, have given rise to proprietary methods to avoid their side effects, and as a result have made unified time harder to maintain. Such operational inconsistency is another reason the General Conference is moving to make UTC continuous.
Systems based on Earth's rotation will need to adapt
Even if leap seconds are abolished, UT1 will remain important in fields such as astronomical observation that need to know Earth's orientation accurately. Resolution 5 explains that the difference between UT1 and UTC can be obtained from the navigation messages of global navigation satellite systems (GNSS) and from internet distribution by bodies such as the IERS. The BIPM was also asked to continue monitoring observations and future predictions of the difference between the two. In other words, the measurement of Earth's rotation will not stop; instead, the arrangement will shift to one in which systems that need that information refer to it individually.
Equipment and software that have so far used UTC as an approximation of UT1 will need to be able to handle the difference correctly even as it grows. Meanwhile, for uses that need a continuous common time, such as communications and data processing, there will be no need to adjust clocks to match Earth's rotation. Separating the maintenance of a common time from the use of Earth-rotation information makes it possible to manage time in the way best suited to each use.
There is also a technical reason the new tolerance was set at one hour rather than left unlimited. In Section 3 of the CCTF technical report, options of one minute, one hour, and no upper limit were considered. People involved in systems that distribute the difference between UT1 and UTC reportedly asked for a finite upper limit so that the number of bits needed to record and transmit the value could be determined. The decision substantially reduces how often corrections would be needed while also making clear the maximum value that equipment handling the time difference must anticipate.
Separately, the coding scheme used to broadcast the difference between UTC and UT1 by radio will need to be updated. Under transitional measures of the International Telecommunication Union (ITU) cited in the resolution, where early replacement of existing equipment is difficult, a grace period is allowed for moving to radio distribution of the expanded time difference until 2035, and 2040 at the latest. This does not, however, postpone the making of UTC continuous itself, planned for May 20, 2027. The change in how UTC is operated and the updating of equipment that distributes the time difference will proceed as separate processes.
The General Conference also discussed "redefining the second" using optical clocks. It asked that a proposed new definition and a proposed implementation date be prepared for the 2030 meeting, but agreement has not yet been reached on matters such as which atomic species to adopt. Abolishing leap seconds changes how the world's common time is operated, and it is a separate effort from redefining the second, which would change the standard that fixes the length of a second. Work on the latter will continue in order to meet scientific requirements.
What will matter in the 2027 transition is whether time-distribution systems can reliably deliver continuous UTC, and whether devices and software that need Earth-rotation information can correctly handle the growing difference from UT1. Because of this decision, there will be no need for the time being to put a one-second step into the world's common clock. At the same time, observation and information distribution for handling Earth's rotation with high accuracy will remain necessary. Whether each system can properly reflect this division of roles will be the key to a successful move to a new way of keeping time without leap seconds.