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Leap seconds would be replaced by a continuous UTC time scale allowed to drift farther from Earth-rotation time (UT1). Draft Resolution C before the 28th General Conference on Weights and Measures (CGPM) proposes permitting an absolute UT1−UTC difference of up to 3,600 seconds (one hour), instead of inserting a one-second step whenever the difference approaches 0.9 seconds. UT1−UTC would still be published for applications that need precise knowledge of Earth’s rotation.
The one-hour limit and a proposed effective date of 20 May 2027 are not final as of 13 October 2026: the CGPM meeting is under way and must agree to the draft. If adopted, the change would remove discontinuities from the world’s main reference time without stopping anyone from measuring or using Earth-rotation data.
The short answer: continuous UTC, with UT1−UTC reported separately
There is no proposed replacement gadget, new kind of second, or alternative atomic clock. The replacement is a rule change. UTC would continue at the same uniform rate as International Atomic Time (TAI), while the permitted gap between UTC and UT1 would become much larger. Rather than adding or removing a second, international time services would let that gap accumulate and publish its value for specialist users.
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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteThat distinction matters because UTC and UT1 answer different questions. UTC is the internationally coordinated reference used to define civil time and synchronize systems. UT1 describes the angle of Earth’s rotation. Earth does not rotate at a perfectly constant rate, so UT1 wanders relative to atomic time. Leap seconds currently make UTC track that wandering closely. The proposal keeps the measurements but stops forcing UTC to follow them in one-second jumps.
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Three time scales you need to understand
International Atomic Time (TAI)
TAI is the continuous atomic time scale produced by the International Bureau of Weights and Measures (BIPM). It advances uniformly according to an international ensemble of atomic clocks. It has no leap seconds.
Coordinated Universal Time (UTC)
UTC is the international reference and the basis for civil time in most countries. It runs at the same rate as TAI but differs from TAI by an integral number of seconds. That whole-second offset changes when a leap second is introduced under the current system.
Universal Time 1 (UT1)
UT1 is derived from observations of Earth’s angular rotation. It is the scale used when the orientation of Earth itself matters, such as astronomical pointing, satellite operations and geodesy. The published quantity UT1−UTC tells users how far the reference time is from Earth-rotation time.
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Since 1972, leap seconds have kept UTC close to UT1. Under the rule summarized in the 2022 CGPM resolution, the responsible authorities predict the value of UT1−UTC and schedule a leap second when the difference is expected to approach 0.9 seconds. A positive leap second creates a minute with a 60th second; a negative leap second would remove a second from a minute. The date is announced in advance so national laboratories, network operators and software developers can prepare.
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This arrangement keeps clock time and the rotating Earth nearly aligned, but it introduces a discontinuity into an otherwise uniform reference scale. Different systems have handled that discontinuity in different, uncoordinated ways. Some stop or repeat a timestamp, some smear the extra second over a longer interval, and others apply a special correction. The 2022 resolution identifies those divergent implementations as a synchronization risk for critical infrastructure.
What Draft Resolution C would change
BIPM’s 2026 explanation of Draft Resolution C proposes a maximum absolute UT1−UTC difference of 3,600 seconds, or one hour. The draft would therefore discontinue leap seconds and allow UTC to remain continuous for centuries, according to BIPM’s description. It would not redefine the SI second, replace atomic clocks or abolish UT1.
| Aspect | Current arrangement | Draft proposal |
|---|---|---|
| UTC continuity | UTC can contain a one-second insertion or deletion. | UTC would run continuously without leap-second steps. |
| Relationship to Earth rotation | UTC is kept close to UT1. | UTC may drift farther from UT1; the offset remains available to users. |
| Operational threshold | A leap second is scheduled when UT1−UTC is predicted to approach 0.9 seconds. | No recurring leap-second trigger; the proposed absolute limit is 3,600 seconds. |
| Effect on TAI | UTC differs from TAI by an integral number of seconds, changed by leap seconds. | UTC would retain the same rate as TAI and continue to differ by an integral number of seconds. |
| Status | Existing international practice. | Draft text awaiting CGPM agreement as of 13 October 2026. |
| Proposed effective date | Not applicable. | 20 May 2027, only if the draft is agreed. |
“Maximum absolute difference” means the limit applies in either direction: UT1 could be up to one hour ahead of UTC or up to one hour behind it. The exact operational procedures for distributing UTC and Earth-orientation information would still be handled through the international timekeeping system; the central change is that UTC would no longer be stepped to stay within roughly a second of UT1.
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Discontinuous timestamps are difficult for computers
Most software assumes that a timestamp sequence is monotonic: the next timestamp is later than the previous one by a predictable amount. A leap second violates that assumption. A system can encounter a timestamp that occurs twice, a 60th second that ordinary date-time libraries do not represent, or a missing second if a negative leap second is ever used. Distributed databases, event logs and control systems can disagree about which interpretation is correct.
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Infrastructure has no single implementation method
The 2022 resolution says operators have adopted different methods for handling leap seconds and have not always coordinated them. A network that inserts the extra second exactly at the announced instant can temporarily disagree with one that gradually smears the adjustment. Those differences can trigger false timeouts, ordering errors or loss of synchronization. The resolution specifically names Global Navigation Satellite Systems, telecommunications and energy transmission as areas where such failures could be serious.
A negative leap second would be especially unfamiliar
Positive leap seconds have been used, but a negative leap second has never been inserted. BIPM has reported that specialists have considered the possibility and that either sign creates synchronization risks. A negative leap second is not scheduled or inevitable; it is one reason authorities prefer a continuous reference scale over waiting for an unprecedented deletion to be required.
When could the change take effect?
The 2022 CGPM resolution set the direction: develop a higher permitted UT1−UTC difference and an implementation plan by or before 2035, with the goal of keeping UTC continuous for at least a century. In 2026, BIPM presented Draft Resolution C with the one-hour maximum and 20 May 2027 as the proposed effective date. The International Committee for Weights and Measures (CIPM) endorsed the update for presentation to the CGPM, but that endorsement is not the same as adoption.
The 28th CGPM is scheduled for 13–15 October 2026. On 13 October, the meeting had opened, so the proposal must be described as conditional. Until delegates agree to the resolution and any associated implementation decisions, leap-second practice remains the operative arrangement.
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What would change for ordinary clock users?
Daily civil time would look the same
Phones, computers and public clocks would still display UTC-derived civil time and local time-zone offsets. There would be no visible one-hour jump when the policy begins. The proposal concerns the long-term relationship between UTC and Earth rotation, not the length of an hour or the way time zones are legislated.
UTC timestamps would become easier to process
Systems that store, compare and transmit UTC would no longer need a special leap-second branch. A continuously increasing timeline avoids duplicated or exceptional seconds at the insertion point. This is particularly useful for distributed services whose logs, authentication windows and ordering rules depend on monotonic time.
The Sun would slowly move away from clock noon
Because Earth’s rotation is not uniform, allowing UT1−UTC to grow means UTC would gradually become less closely aligned with the position of the Sun. The proposed limit is one hour, not an immediate one-hour shift. The offset would accumulate over a long period and would be monitored through the published UT1−UTC value. Civil time would remain stable while the Earth-rotation relationship changed gradually.
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What would not change?
- Earth-rotation measurements: UT1 would continue to be determined and distributed.
- Atomic time: TAI would remain the continuous atomic scale, and UTC would continue at its rate.
- Time-zone law: Countries would still decide their civil time and daylight-saving rules.
- Satellite and astronomical calculations: Specialists would still use Earth-orientation parameters, including UT1−UTC, when their applications require them.
- Existing historical timestamps: Past leap seconds would remain part of the historical relationship between UTC and TAI.
Who needs to pay attention?
Telecommunications and network operators
Operators would gain a reference scale without a sudden second being inserted into network clocks. They would still need to obtain authoritative UTC and, where required, Earth-orientation data. Systems that currently test leap-second handling should be updated to reflect the eventual international specification rather than simply deleting those tests.
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Global Navigation Satellite Systems
GNSS services use carefully defined time scales and Earth-orientation information. A continuous UTC policy could simplify interfaces that exchange UTC timestamps, but each constellation and receiver specification would determine how the change is exposed. Users should follow the notices and interface documentation issued by the relevant service, not assume that every GNSS signal will change on the same date.
Energy transmission and industrial control
Power-grid and industrial systems depend on precise ordering and synchronized measurements. Removing a one-second discontinuity addresses one class of failure, but it does not remove the need for redundant timing sources, holdover clocks, monitoring and a documented response to loss of synchronization.
Software and database developers
Developers should distinguish UTC from monotonic elapsed-time clocks. A policy change does not make a wall clock suitable for measuring durations, and it does not eliminate historical data containing leap-second conventions. Libraries, protocol specifications and database engines will need to document how they represent UT1−UTC and any transition date adopted by the CGPM.
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The proposal separates two jobs that are often conflated. UTC would provide a stable, continuous label for events. UT1−UTC would provide the correction needed when an application cares about Earth’s orientation. An observatory, satellite operator or geodesist could apply the published offset to relate a UTC timestamp to the rotating Earth, just as users already obtain Earth-orientation parameters today.
This is not the same as ignoring rotation. It is closer to keeping a reliable reference clock and publishing a continuously changing conversion to an irregular physical process. The conversion would become larger over time, but it would remain measurable and available.
Questions the proposal does not answer by itself
- The exact distribution format: Draft Resolution C establishes the policy direction and proposed limit; technical services must specify how offset data are disseminated and consumed.
- Every system’s migration plan: Individual GNSS providers, laboratories, telecom operators and software vendors must publish their own compatibility guidance.
- The final date: 20 May 2027 applies only if the CGPM agrees to the draft and associated implementation arrangements.
- The future size of the offset on a particular day: Earth’s rotation varies, so the UT1−UTC value cannot be inferred from the one-hour limit alone.
How to prepare if you operate time-dependent systems
- Inventory your clock assumptions. Find code and hardware that assumes every minute has 60 seconds, compares wall-clock values for elapsed time, or treats UTC as identical to UT1.
- Separate clock labels from durations. Use a monotonic clock for timeouts and elapsed intervals; reserve UTC for timestamps that must be mapped to civil or international reference time.
- Document leap-second behavior. Record whether each component inserts, repeats, smears or ignores a leap second so historical data can be interpreted correctly.
- Track authoritative notices. Follow BIPM, national metrology institutes and the relevant GNSS or protocol authority for the final resolution, effective date and data formats.
- Test both eras. Preserve tests for historical leap-second records while adding tests for continuous UTC and for consuming UT1−UTC values that may be much larger than one second.
- Plan for outages. Keep holdover and cross-check procedures: abolishing leap seconds removes a discontinuity but does not eliminate clock drift, network failures or bad timing data.
Bottom line
If the CGPM approves Draft Resolution C, leap seconds will be replaced by allowing UTC to remain continuous while the permitted UT1−UTC difference expands to as much as 3,600 seconds. Earth’s rotation will still be measured, and its relationship to UTC will still be published. The proposed 20 May 2027 start date and one-hour limit are decisions awaiting agreement, not settled facts as of 13 October 2026.
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