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No, Earth’s days are not becoming longer or shorter. The change is to civil timekeeping. On 20 May 2027, Coordinated Universal Time (UTC) will become continuous: clocks will stop receiving routine leap-second adjustments intended to keep them within one second of Earth’s irregular rotation. Earth will continue rotating at a variable rate, and the Sun will still determine observed sunrise, sunset and apparent solar time.
What was decided
At its 28th meeting in October 2026, the General Conference on Weights and Measures (CGPM) adopted Resolution 5 on the continuity of UTC. The resolution says continuous UTC becomes effective on 20 May 2027 and permits the absolute difference between UTC and UT1—the time scale based on Earth’s rotation—to reach 3,600 seconds, or one hour. The exact wording is in the BIPM Resolution 5 (2026).
UTC will remain generated from atomic time. The policy change removes the requirement for occasional one-second steps, allowing atomic civil time to drift gradually away from rotational time over centuries.
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What is changing—and what is not
| Question | Answer after 20 May 2027 |
|---|---|
| Will a normal day stop being 24 hours? | No. “Day” can mean a civil 24-hour calendar day, an Earth-rotation day, or the daylight interval between sunrise and sunset. The resolution changes only the relationship between UTC and UT1. |
| Will Earth rotate at a fixed rate? | No. Earth’s rotation will continue to vary because of geophysical and atmospheric effects. |
| Will sunrise and sunset suddenly move? | No. Local sunrise and sunset follow Earth’s rotation, your location and the season. There is no one-hour jump in daylight on the start date. |
| Will leap seconds continue routinely? | No. UTC will be continuous rather than being periodically stepped to stay within one second of UT1. |
| How far may UTC and UT1 eventually separate? | The new maximum permitted difference is 3,600 seconds (one hour), rather than the former one-second target. |
Why leap seconds are being retired
Irregular steps are difficult for software
UTC is based on atomic clocks, while UT1 follows Earth’s changing spin. Under the old system, a leap second was inserted when needed to keep UTC and UT1 within one second. A one-second clock step is unusual behavior for operating systems, databases, distributed services and communication protocols that normally assume time advances continuously.
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Leap seconds had been inserted 27 times since the 1970s, with the last in 2016, according to a contemporaneous report by The Telegraph carried by AOL. The same report said the 2012 leap-second event was associated with crashes or other problems reported by Mozilla, Reddit, Foursquare, Yelp, LinkedIn, StumbleUpon, Linux and Java systems. Those are reported implementation incidents, not the result of a controlled comparative study. The report is available at AOL/The Telegraph (15 October 2026).
A negative leap second would be harder still
A positive leap second adds 23:59:60. A negative leap second would remove a second from a UTC day, creating a sequence that skips a label. No negative leap second has ever been applied. The CGPM resolution records a 2025 workshop estimate from the Consultative Committee for Time and Frequency and the International Earth Rotation and Reference Systems Service that the probability of needing one could reach 30% by 2035. The concern is not that a negative leap second is physically impossible, but that many systems are not designed for either kind of discontinuity.
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Metrologist Noel Dimarcq of the Côte d’Azur Observatory called the decision “a historical moment, a decisive step for UTC” and described it as an international reference time scale continuous for the next millennia. Dr Christian Bizouard of the International Earth Rotation and Reference Systems Service said, “When we apply leap seconds, there are always errors.” Both statements were reported by AOL/The Telegraph.
How the old and new systems differ
| Comparison | Previous UTC practice | Continuous UTC from 2027 |
|---|---|---|
| Synchronization target | Keep UTC within one second of UT1. | Allow UTC–UT1 separation up to one hour. |
| Operational behavior | Occasional one-second insertions or removals. | No routine leap-second steps. |
| Main technical risk | Systems mishandle an unexpected repeated or skipped second. | Long-term civil time gradually diverges from solar-rotation time. |
| Most affected users | Systems requiring sub-second coordination and robust leap-second handling. | Organizations that must monitor UTC–UT1 over long periods or explain the growing civil/solar offset. |
What the change means for people and technology
Everyday clocks
Phones, computers, wall clocks and calendars will still display familiar 24-hour civil time. Most people will not see a visible change on 20 May 2027. A radio-controlled atomic clock illustrates how consumer devices can receive atomic-time-based civil time; it does not measure Earth’s rotation or predict sunrise.
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Critical infrastructure
Transportation, energy distribution, telecommunications and space projects often coordinate events across networks and locations. For these users, removing sporadic one-second steps makes timestamp handling more predictable. Operators still need a reliable UTC source, documented clock discipline and monitoring of the UTC–UT1 difference.
Sub-second applications
Trading, scientific instruments, navigation, satellite operations and other systems that compare measurements to Earth orientation cannot simply discard UT1. They will continue to obtain Earth-orientation information separately while using continuous UTC as the stable civil reference. Engineering documentation should state explicitly whether a timestamp is UTC, UT1 or another time scale.
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- Multiple Display Formats: Large digital wall clock with backlight makes it easy to know the time, two different time and temperature formats (12/24H; /) are available for your convenience and preference
Will solar time eventually drift from clock time?
Yes, slowly. Because UTC will no longer be corrected to follow every fluctuation in Earth’s spin, the average position of the Sun in the sky at a given UTC time will change over long periods. The Telegraph/AOL report estimated an approximate civil-to-solar drift of 25 minutes every 500 years; that is a broad long-term estimate, not a forecast for a particular city or date. The drift is imperceptible in ordinary daily life but matters to future timekeeping conventions and astronomical applications.
What organizations should do before the transition
- Inventory time dependencies. Identify software, devices and services that parse UTC, UT1, leap indicators or the value 23:59:60.
- Separate time scales in data models. Label whether each timestamp is UTC, UT1, local civil time or another reference, rather than treating them as interchangeable.
- Remove assumptions about future leap seconds. Test schedulers, databases, message queues and monitoring systems without relying on a recurring leap-second event.
- Keep Earth-orientation feeds where needed. Scientific, navigation and space operations may still require UT1 and related Earth-orientation data even though civil UTC is continuous.
- Verify vendor and standards updates. Obtain transition guidance for operating systems, timing receivers, telecom equipment and satellite-control software, and test failover behavior before 20 May 2027.
The practical answer
The Sun is not losing its role in defining daylight, and Earth is not being forced to rotate to a new schedule. The international time standard is changing its priority: after 20 May 2027, UTC will favor a continuous, software-friendly atomic timescale and accept a slowly increasing difference from Earth-rotation time, up to one hour. Sunrise, sunset and the physical length of a day remain governed by astronomy, not by the UTC policy.
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