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Earth does not rotate at a perfectly constant rate, but the seconds used by modern clocks are designed to be uniform. Timekeepers bridge that mismatch by keeping civil UTC close to Earth-rotation time, UT1, and occasionally adjusting UTC with a leap second. The adjustment keeps the difference between the two time scales below 0.9 seconds; it does not change the length of the SI second.
As of 13 October 2026, the latest cited IERS announcement says no leap second will be added at the end of December 2026. That is a decision about the next announced opportunity, not a guarantee about future dates.
Why Earth’s rotation matters to clocks
For much of human history, timekeeping followed the apparent motion of the Sun. The cycle from one solar noon to the next provided a practical basis for dividing the day. Modern timekeeping has a different foundation: atomic standards realize highly uniform seconds, and UTC—the civil time scale used for everyday clock time—is based on atomic time.
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The difficulty is that the planet used to define the solar day does not rotate like a perfectly regulated clock. The International Earth Rotation and Reference Systems Service (IERS) describes Earth’s rotational speed as highly variable because of tides, changes in weather, oceans, and other geophysical effects. As a result, a rotation-based day is not always exactly 86,400 SI seconds.
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That difference matters whenever a time scale must describe both a regular clock and Earth’s actual orientation. Astronomers and geodesists need to know where Earth is in its rotation; civil clocks need a stable, coordinated standard. UT1 and UTC serve those different needs, and leap seconds manage their growing separation.
UT1, UTC, TAI and TT: four time scales with different jobs
These names are not interchangeable. The key distinction is whether a scale follows observed Earth rotation or atomic time, and whether its purpose is civil timekeeping or calculation.
| Time scale | Basis | Main purpose | Relationship to leap seconds |
|---|---|---|---|
| UT1 | Observed Earth rotation | Represents Earth’s rotational orientation; it is mean solar time at longitude zero. | It is compared with UTC to track the difference between rotation-based and civil time. |
| UTC | Atomic-clock-based civil time | The coordinated time scale used for civil clock time. | Leap seconds adjust it to keep it close to UT1. |
| TAI | Atomic time | International Atomic Time, the atomic scale used to express UTC’s offset. | UTC is related to TAI by the accumulated leap-second offset. |
| TT | Derived from TAI | A time argument used in some astronomical models. | IERS gives the conventional relation TT = TAI + 32.184 seconds; TT is not a substitute name for UTC. |
At the time of IERS Bulletin C 72, UTC−TAI was −37 seconds, effective from 1 January 2017 “until further notice.” This is the stated offset between those two scales, not a statement that a civil clock is 37 seconds wrong. The scales have different definitions and uses.
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Time arguments also matter in technical data. IERS Earth-orientation data in Bulletins A and B use 0h UTC as the time argument, while some celestial-mechanics models require TT. Substituting one scale for another without checking the specification can introduce a timing error into calculations even when both values are expressed in seconds.
How UT1−UTC connects clock time to Earth’s orientation
The expression UT1−UTC is the difference between Earth-rotation time and coordinated civil time. It tells specialists how far the rotation-based measure has moved relative to UTC. Because Earth’s spin varies, the difference changes over time; it is observed and monitored rather than inferred from a fixed assumption that the planet always speeds up or slows down at one steady rate.
UT1 is derived from observations of Earth’s rotation. One important measurement method uses very-long-baseline interferometry (VLBI): radio telescopes observe distant radio sources such as quasars, and the observations help determine Earth’s orientation and UT1. The measurements are not simply a reading from one clock; they are part of an Earth-orientation monitoring system.
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The IERS Earth Orientation Centre monitors long-term Earth-orientation parameters, publishes time-dissemination information, and announces leap seconds. The U.S. Naval Observatory’s IVS VLBI Analysis Center analyzes daily Intensive sessions, monitors UT1−UTC, and reports measured values to IERS and the International VLBI Service for Geodesy & Astrometry. This division connects observations to the official time-scale decisions.
What a leap second does—and what it does not do
A leap second adjusts UTC in relation to Earth’s rotation. Under the current system, the IERS glossary says leap seconds are introduced so that the absolute value of UT1−UTC stays below 0.9 seconds. The 0.9-second figure is a bound on the difference between the scales; it is not the length of a leap second, nor does it mean every adjustment is 0.9 seconds.
A positive leap second is one additional second. The adjustment is associated with a UTC month boundary, conventionally at the end of June or December in the current announcement format. A negative leap second is possible in principle, but the available announcement does not say one is scheduled. The underlying SI second is not redefined or stretched to make the correction: UTC’s relationship to Earth rotation is what is adjusted.
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In plain terms, atomic time keeps the second consistent while UT1 tracks the planet’s changing rotation. If their difference approaches the operational limit, IERS can announce a leap-second adjustment to UTC. This avoids forcing the atomic second to imitate the variable rotation of Earth.
Who measures the difference and decides whether to adjust UTC?
Measurements and decisions are related but separate. Observatories and analysis centers contribute Earth-rotation observations; IERS monitors Earth-orientation information and is responsible for deciding whether a leap second should be introduced. Its Bulletin C is the formal announcement channel for leap-second decisions.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesThe IERS document on leap seconds reproduces the relevant ITU recommendation: “The IERS should decide upon and announce the introduction of the leap-second, such as an announcement to be made at least eight weeks in advance.” In practice, the reader-facing source to check for an announced decision is the latest Bulletin C, rather than a prediction based on an assumed schedule.
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Will there be a leap second at the end of 2026?
No. IERS Bulletin C 72, issued in Paris on 6 July 2026, states: “NO leap second will be introduced at the end of December 2026.” It also states that UTC−TAI remains −37 seconds from 1 January 2017 until further notice.
This answers the announced December 2026 date only. It should not be read as a permanent policy or as a prediction for later dates: Earth’s rotation is variable, and leap-second decisions are based on monitoring. For any future boundary, consult the most recent IERS Bulletin C.
What this means for everyday clocks and technical systems
For most people, UTC is encountered indirectly through clocks and systems that coordinate time across locations. The distinction between UTC and UT1 usually matters more in astronomy, geodesy, satellite or Earth-orientation work, and software or data pipelines that require a precise time argument. A system that only needs ordinary civil time may not need to expose UT1; a calculation tied to Earth’s orientation may.
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Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →- If you need civil time: use UTC or a local time derived from it as appropriate to the application. Do not treat UT1 as another spelling of UTC.
- If you need Earth’s orientation: use the relevant UT1 or IERS Earth-orientation data, and check which time argument the product specifies.
- If you are interpreting an offset: keep the sign and scale labels attached. UTC−TAI = −37 seconds is not the same quantity as UT1−UTC.
- If you need to know whether a leap second is announced: check the current IERS Bulletin C rather than assuming one occurs on a regular cadence.
Leap seconds can require careful handling in software because a civil-time adjustment is not simply a change in the SI second. The particular way a clock, operating system, database, or service represents or distributes an announced adjustment depends on that system; the cited IERS and USNO material establishes the time-scale rules and monitoring roles, not behavior for every implementation. Systems with strict timestamp requirements should follow their own time-source and data-format specifications.
Common misunderstandings to avoid
- “A day is always exactly 86,400 seconds.” UTC civil days are defined as 86,400 seconds in the USNO description, but a rotation-based day does not always equal that number of SI seconds.
- “Leap seconds make the SI second longer or shorter.” They correct UTC’s relationship to Earth rotation; they do not redefine the SI second.
- “The 0.9-second threshold is the leap-second size.” It is the limit on the absolute UT1−UTC difference under the current system. A positive leap second is one second.
- “Leap seconds arrive on a predictable schedule.” They depend on observed Earth rotation and an IERS decision; the adjustment is not a fixed annual event.
- “A notice about one date settles future dates.” Bulletin C 72 decides that there will be no leap second at the end of December 2026. Future operational status requires a later announcement.
The practical distinction
Earth’s rotation supplies the physical reference for UT1, while atomic standards supply the regular seconds used by UTC and TAI. Because the rotation varies, the two kinds of time do not remain perfectly aligned. UT1−UTC measures that separation; IERS monitors it and announces a leap second when needed to keep its absolute value below 0.9 seconds. The result is a civil time scale that remains close to Earth’s rotation without making the second itself irregular.
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