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The General Conference on Weights and Measures has decided that UTC will become continuous on May 20, 2027, ending the insertion of leap seconds. The new limit for the difference between UTC and Earth-rotation time, UT1, will be one hour; the decision is intended to reduce risks to systems that rely on synchronized clocks.
The General Conference on Weights and Measures (CGPM) has set May 20, 2027 as the date when Coordinated Universal Time (UTC) becomes continuous, ending the use of leap seconds to keep it closely aligned with Earth’s rotation. Under Resolution 5, UTC may differ from Earth-rotation time, known as UT1, by as much as 3,600 seconds, or one hour—a change intended to protect systems that depend on uninterrupted timekeeping.
The resolution was adopted at the 28th CGPM meeting in 2026. It establishes a specific implementation date, replacing the previous plan from a 2022 resolution to increase the permitted UTC–UT1 difference in or before 2035. The BIPM, which produces UTC, is asked to work with relevant international organizations to inform users and support implementation.
UTC is produced by the International Bureau of Weights and Measures (BIPM) at the same rate as International Atomic Time (TAI), differing from it by an integral number of seconds. Leap seconds have been used to keep UTC aligned within a specified limit of UT1, a time scale based on Earth’s rotation. From the effective date, UTC will no longer receive leap-second adjustments; its separation from UT1 can instead grow within the new one-hour ceiling.
The resolution cites the risk that a negative leap second could be needed as Earth’s rotation changes. A 2025 workshop organized by the Consultative Committee for Time and Frequency and the International Earth Rotation and Reference Systems Service (IERS) estimated a 30% probability of one by 2035. No negative leap second has previously been applied, and the CGPM says systems may be poorly prepared for one.
Why Continuous UTC Matters to Infrastructure
Removing leap-second insertions is intended to make a widely used international time reference more predictable for computer networks, telecommunications, energy distribution, transportation and space projects. These systems often coordinate operations across many devices, and even a one-second change can require software and operational rules to handle the adjustment consistently.
The CGPM resolution says some global service providers already use different methods to manage leap seconds, including approaches designed to avoid a sudden time step. It warns that such methods can create inconsistencies between systems and disseminated time. A shared continuous UTC is intended to reduce that source of divergence, although organizations will still need to account for the growing difference between UTC and Earth-rotation time where their work requires UT1.
The change also matters beyond technical infrastructure. UTC is the recommended time scale for international reference and is the basis of civil time in most countries. The resolution says delays in changing the system could lead some users to switch to other reference scales, including GNSS time, weakening the common international standard. The decision aims to retain UTC as that shared reference while changing how it relates to Earth’s rotation.
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From Leap-Second Limits to a Fixed Date
The policy shift follows a series of international decisions. In 2018, the 26th CGPM confirmed UTC’s relationship to TAI and its role as the recommended international reference time scale. In 2022, the 27th CGPM decided that the maximum permitted difference between UT1 and UTC would be increased in or before 2035, with the aim of preserving UTC’s continuity for at least a century, and asked the international metrology community to prepare a further resolution.
That earlier decision reflected the tension between two goals: keeping civil and technical time connected to Earth’s rotation, and avoiding clock adjustments that can disrupt digital systems. The new resolution sets the limit at one hour, which it says supports UTC’s long-term continuity for several centuries. Users needing UT1 information can obtain it through GNSS navigation messages and services provided by the IERS, national metrology institutes and other providers.
Implementation also involves radio time signals. The International Telecommunication Union’s radiocommunication sector has been studying possible updates to how the UT1–UTC difference is transmitted. A 2023 World Radiocommunication Conference resolution allows a transition period, potentially extending to 2040 for cases where existing equipment cannot be replaced earlier. The 2026 CGPM resolution asks the BIPM to continue collaboration with the ITU, IAU, IUGG, IERS and affected organizations.
“Continuous UTC will become effective on 20 May 2027.”
— The 28th General Conference on Weights and Measures, Resolution 5
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Remaining Work for Time Signal Users
The decision sets the policy and date, but the resolution also recognizes that implementation details remain in progress. The ITU-R is studying updates to radio transmission codes for the UT1–UTC difference, and some existing equipment may need a transition period. The source resolution does not specify a complete schedule for individual sectors, devices or national services.
It is also not clear from the resolution how quickly different operators will adapt their systems or what costs they will face. The CGPM refers to investment needs and says preparation for a negative leap second could be comparable in scale to preparations for the millennium bug, but that is the resolution’s characterization rather than a detailed cost estimate. The projected 30% probability by 2035 is likewise an estimate cited from the 2025 workshop, not a forecast that a negative leap second will occur.
UTC’s increasing offset from UT1 will also require communication and planning for users who depend on Earth-rotation time. The resolution points to existing channels for accessing UT1 information, but does not set out specific operating procedures for every such user. The BIPM is tasked with monitoring the offset and reporting its evolution to the International Committee for Weights and Measures (CIPM).
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Preparations Before May 2027
Before May 20, 2027, the BIPM is to continue coordinating with international organizations and support users as they apply the decision. It is also tasked with monitoring the UT1–UTC difference, including its estimated evolution and modelling as determined by the IERS, and regularly informing the CIPM about changes.
The next practical milestones include work by the ITU-R on radio time-signal formats and guidance for organizations using equipment that cannot be updated quickly. The 2023 radio conference resolution provides for a transition period in certain cases through no later than 2040. The 2026 resolution does not identify individual system upgrades or publish a sector-by-sector implementation timetable.
For users, the immediate change is not that clocks will jump by an hour in 2027. Rather, leap seconds will cease, and the difference between UTC and UT1 will be allowed to grow over time, up to the new limit. How that affects specific services will depend on their need for Earth-rotation time and the systems they use to distribute or interpret time signals.
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Key Questions
When will UTC stop using leap seconds?
The 28th CGPM’s Resolution 5 says continuous UTC takes effect on May 20, 2027. The resolution ends future leap-second adjustments from that date.
Will clocks change by one hour in May 2027?
No. The one-hour figure is a maximum permitted difference between UTC and UT1, not a scheduled clock change. The gap is allowed to grow over time as Earth’s rotation and atomic time differ.
What are UTC and UT1?
UTC is the international reference time scale produced by the BIPM and used as the basis of civil time in most countries. UT1 is a time scale related to Earth’s rotation; leap seconds have historically kept UTC close to it.
Why did the CGPM decide to end leap seconds?
The resolution cites risks to digital and critical infrastructure from leap-second insertions, especially a negative leap second, which has never been used. It also points to inconsistent ways different providers currently handle leap seconds.
Will users still be able to find the UT1–UTC difference?
Yes. The resolution notes that UT1 information is available through GNSS navigation messages and services provided by the IERS, national metrology institutes and other providers. Radio-signal transmission methods are still under study.
Source: hn
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