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The 28th General Conference on Weights and Measures decided that continuous UTC will take effect on May 20, 2027, ending future leap-second adjustments. The permitted difference between UTC and Earth-rotation time UT1 will rise to one hour; the change is intended to reduce risks for systems that depend on synchronized time.
The General Conference on Weights and Measures (CGPM) has decided that Coordinated Universal Time will become continuous on May 20, 2027, ending adjustments through leap seconds. Under Resolution 5 of its 28th meeting, the permitted difference between UTC and Earth-rotation time UT1 will increase to as much as one hour, a change intended to reduce disruption risks for systems that rely on synchronized time.
The resolution says continuous UTC will take effect on the specified date and sets a maximum absolute difference of 3,600 seconds between UT1 and UTC. UTC will remain the international reference time scale produced by the International Bureau of Weights and Measures (BIPM); the change concerns how it is kept aligned with Earth’s rotation. It does not change the definition of a second or mean that civil clocks will suddenly move by an hour in 2027.
Leap seconds have historically been inserted to keep UTC close to UT1, a time scale that reflects the Earth’s rotation. The CGPM’s decision removes that continuing adjustment requirement. Organizations and users that need to know Earth’s rotation relative to UTC can use published UT1-UTC information, including data distributed through satellite navigation messages and services from the International Earth Rotation and Reference Systems Service (IERS), national metrology institutes and other providers.
The resolution directs the BIPM to work with the International Telecommunication Union, the International Astronomical Union, the International Union of Geodesy and Geophysics, the IERS and other affected organizations. It also asks the BIPM to monitor the evolving UT1-UTC difference and report regularly to the International Committee for Weights and Measures (CIPM). These steps are intended to support implementation and keep users informed.
Why Continuous UTC Matters to Systems
The change is meant to make UTC more predictable for technical systems that coordinate activity across networks. The CGPM resolution identifies transportation, energy distribution, telecommunications and space projects as areas where industrial users and critical infrastructure rely on continuous time. Leap-second handling can require software changes, and different ways of managing an inserted second can leave systems temporarily using inconsistent time.
The resolution cites a particular concern about a possible negative leap second, which would subtract rather than add a second. Such an adjustment has never been applied. A BIPM-cited expert workshop estimated that the probability of one would rise to 30% by 2035. The resolution says critical infrastructure is largely unprepared for that possibility and that preparing for it could involve substantial costs for industries dependent on synchronized time.
Allowing UT1 and UTC to differ by up to an hour makes uninterrupted UTC possible while retaining a way to communicate the difference for users who need Earth-rotation time. The decision does not eliminate the value of UT1 for astronomical or other applications; it changes the relationship between that measure and the civil time standard. The practical effect for most people is expected to be limited: local time will continue to be set by national rules, including time zones and daylight-saving changes.
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How the Leap-Second Policy Changed
UTC is maintained by the BIPM and has the same rate as International Atomic Time (TAI), differing from it by an integral number of seconds. Since 1972, leap seconds have been used to keep UTC aligned within a prescribed range of UT1, which tracks the Earth’s rotation. Because Earth’s rotation varies, the timing of any leap-second adjustment depends on observations and announcements rather than a fixed calendar schedule.
The CGPM had already set the direction for change. In Resolution 4 of 2022, its 27th meeting agreed that the maximum permitted UT1-UTC difference would be increased in or before 2035, with the aim of ensuring UTC’s continuity for at least a century. Resolution 5, adopted at the 2026 meeting, sets the implementation date and the one-hour limit.
The BIPM resolution also refers to international radio-communications work on transmitting UT1-UTC. The International Telecommunication Union’s World Radiocommunication Conference adopted a provision allowing a transition period for systems unable to replace existing equipment earlier, with the possibility of disseminating a larger difference through radiocommunication systems until 2035, but no later than 2040. This is relevant to users who receive time information through older equipment.
““Continuous UTC will become effective on 20 May 2027.””
— The 28th General Conference on Weights and Measures, Resolution 5
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Implementation Details Still Ahead
The resolution fixes the start date and maximum UT1-UTC difference, but some practical arrangements remain subject to work by the organizations involved. The BIPM is tasked with coordinating with international bodies, informing users, monitoring the difference and reporting its evolution. The resolution does not provide a detailed implementation schedule for every industry or national time service.
It is also not yet clear how quickly all radio systems and other legacy equipment will be updated to handle the larger permitted offset. The ITU provision recognizes that some existing equipment may not be replaced promptly and sets an outer transition limit of no later than 2040 for specified radio dissemination arrangements. The resolution does not describe individual organizations’ costs or the exact software changes they will need.
The cited 30% probability of a negative leap second by 2035 is a model-based estimate from a workshop, not a prediction that one will occur. Since the new policy takes effect in 2027, the immediate outcome is that the planned continuous UTC standard will avoid the need for such a future adjustment, while monitoring of Earth’s rotation and communication of UT1-UTC continue.
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Coordination Before the 2027 Start
Before May 20, 2027, the BIPM is expected to continue coordinating with the ITU, IAU, IUGG, IERS and other affected organizations, as directed by the resolution. The work includes communicating the policy to users and tracking how the difference between UT1 and UTC changes over time. The BIPM is also to keep the CIPM regularly informed.
Radio-communications bodies and infrastructure operators will need to account for the new one-hour limit in relevant systems and information services. Users that require UT1 can continue to consult its published difference from UTC, while UTC itself will no longer be adjusted with leap seconds once the new policy takes effect. The resolution does not specify a separate public rollout milestone beyond the effective date, so further technical guidance and implementation details may emerge from the organizations responsible.
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Key Questions
What did the BIPM and CGPM decide?
The 28th CGPM adopted a resolution setting continuous UTC to take effect on May 20, 2027. The policy ends future leap-second adjustments and permits UT1 to differ from UTC by up to one hour.
Will clocks change by an hour in May 2027?
No. The resolution changes the maximum permitted difference between UTC and Earth-rotation time UT1; it does not direct clocks to jump by an hour. Civil time zones and daylight-saving rules remain separate decisions made under national or regional systems.
Why stop adding leap seconds?
The CGPM says leap-second insertions can create timing inconsistencies and operational risks for systems that rely on continuous synchronization. It highlighted concern about a possible negative leap second, an adjustment that has never been used and for which critical infrastructure may be unprepared.
Can users still find out how UTC differs from Earth’s rotation?
Yes. The resolution says information on UT1-UTC will remain available through sources including satellite navigation messages, IERS services, national metrology institutes and other providers. The BIPM is also directed to monitor the difference.
What remains to be done before the change?
The BIPM and partner organizations are to inform users, coordinate technical arrangements and monitor UT1-UTC. Some radio systems may need time to adapt; the resolution refers to a transition allowance extending to no later than 2040 for certain dissemination arrangements.
Source: hn
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