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Earth’s rotation does not keep a perfectly steady pace, but the seconds used by modern clocks are designed to be uniform. UT1 tracks Earth’s rotation; Coordinated Universal Time (UTC) tracks atomic time and is occasionally adjusted with a leap second to keep it close to UT1. Under the current system, the absolute difference between UT1 and UTC is kept below 0.9 seconds. The latest cited IERS announcement, Bulletin C 72, says no leap second will be introduced at the end of December 2026.
Why atomic clocks and Earth’s rotation do not stay perfectly aligned
For much of human history, people set time by observing the Sun’s apparent movement across the sky. That gives a useful connection between clocks and the day-night cycle, but it does not provide the uniform timing modern science and communications need. Earth’s rotation varies, so a rotation-based day is not always exactly 86,400 SI seconds.
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Modern civil timekeeping instead relies on atomic clocks, which realize a highly uniform second. The second itself is not lengthened or shortened to follow Earth’s changing rotation. The difference arises because a steady succession of atomic seconds and the irregular rotation of the planet do not advance at exactly the same rate.
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This is the central trade-off in modern timekeeping: atomic time provides a stable, uniform unit, while solar time preserves a practical connection to Earth’s day-night cycle. UT1 represents the rotation side of that relationship; UTC is the civil scale adjusted to remain close to it. A leap second corrects their accumulated difference at a specified point rather than changing the definition or duration of the second.
UT1, UTC, TAI and TT: four time scales with different jobs
These time scales are related, but they are not interchangeable. The most useful distinction is whether a scale follows observed Earth rotation or atomic time, and what work it is intended to support.
| 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 determine how closely civil time tracks Earth rotation. |
| UTC | Atomic time, adjusted to stay close to UT1 | The civil time scale used for coordinated clock time. | Leap seconds adjust UTC in relation to Earth rotation. |
| TAI | Atomic time | International Atomic Time, used as the atomic reference from which UTC’s offset is expressed. | UTC is TAI plus the accumulated leap-second offset; IERS Bulletin C 72 states UTC−TAI = −37 seconds from 1 January 2017 until further notice. |
| TT | A time scale related to TAI by a fixed conventional offset | Used as a time argument in some astronomical models. | IERS gives the conventional relation TT = TAI + 32.184 seconds; TT is not a substitute for UTC or UT1. |
In practice, a dataset or calculation must be read in the time scale it specifies. IERS Earth-orientation data in Bulletins A and B use 0h UTC as their time argument, while some celestial-mechanics models require TT. Treating the labels as if they referred to one universal clock can introduce errors into timing or orientation work.
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The difference between UT1 and UTC is especially useful because it connects what astronomers measure about Earth’s orientation with the civil clock scale. UTC is not a second name for UT1: one is atomic-time-based and adjusted, while the other is derived from observed rotation. TAI and TT add two further distinctions. TAI is the atomic reference used to express UTC’s accumulated offset; TT is a conventional time argument used in some astronomical models. When a precise value is published, its time-scale label is part of the information, not a formatting detail.
How scientists measure Earth’s rotation
UT1 is derived from observations of Earth’s rotation rather than from an atomic clock. One important technique is very long baseline interferometry (VLBI): radio telescopes observe distant radio sources such as quasars, and those observations help determine Earth’s orientation and the UT1 value.
The IERS Earth Orientation Centre monitors long-term Earth-orientation parameters, publishes time-dissemination information, and announces leap seconds. The U.S. Naval Observatory (USNO) says its 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 (IVS). This measurement-and-reporting process is why leap-second decisions are based on observed rotation, not a forecasted repeating schedule.
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The institutions have complementary roles: VLBI observations supply measurements used to determine Earth orientation, USNO’s analysis center monitors and reports UT1−UTC values, and IERS monitors Earth-orientation parameters and issues leap-second announcements. This chain matters because the adjustment is tied to a measured relationship between two time scales. It is not simply a calendar correction chosen because a particular year has passed.
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What UT1−UTC tells us
UT1−UTC is the difference between Earth-rotation time and civil UTC. It is the relevant quantity for describing whether UTC remains sufficiently close to the rotation of Earth. A positive or negative value indicates the direction of the difference under the stated subtraction; the important operational point is its magnitude.
IERS’s glossary says leap seconds are used so that the absolute value of UT1−UTC stays below 0.9 seconds. That 0.9-second figure is a bound on the relationship between the two time scales. It is not the size of every leap second, nor a statement that Earth’s day differs from 24 hours by 0.9 seconds.
In other words, the threshold describes how far apart the time scales may be allowed to drift under the current system. It should not be read as a prediction that the next leap second will occur when the difference reaches exactly 0.9 seconds, or as a measure of a daily change in Earth’s rotation. The relevant values are observed and monitored, and IERS announces an adjustment when one is decided.
How a leap second adjusts UTC
A leap second is a one-second adjustment to UTC at a month boundary, conventionally at the end of June or December in the current announcement format. Its purpose is to correct the relationship between civil clock time and Earth rotation. It does not redefine the SI second or make atomic clocks run faster or slower.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsA positive leap second adds one second to UTC’s sequence. In a UTC clock display, the additional second is conventionally labeled 23:59:60 at the end of the relevant day before midnight. The calendar day containing that adjustment has an extra second relative to an ordinary day. A negative leap second is possible in principle; it would remove a second from the UTC sequence rather than add one. The adjustment direction and date depend on IERS decisions, not on a recurring schedule.
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This mechanism can seem counterintuitive: UTC is atomic-time-based, yet it is occasionally adjusted because its purpose is not to be an entirely uninterrupted atomic count. It is the coordinated civil scale designed to remain close to Earth’s rotation. TAI preserves the atomic-time reference without this civil-time adjustment, while UT1 represents the observed rotation.
The phrase “leap second” can also be misleading if it suggests that the SI second itself changes. It does not. A positive leap second is one second long; what changes is UTC’s sequence at the boundary so that UTC does not move too far from UT1. A negative leap second, if introduced, would likewise be an adjustment to the sequence, but in the opposite direction. The official rule concerns the difference between time scales, not a variable-length atomic second.
Who decides, and how the announcement works
IERS is responsible for deciding whether to introduce a leap second and announcing the decision through Bulletin C. The IERS description reproduces an ITU recommendation that an announcement should be made at least eight weeks in advance. That lead time is intended to give time services and other affected systems notice of an upcoming adjustment; it does not mean the adjustment itself can be predicted years ahead.
- Observe rotation. Earth-orientation measurements, including VLBI observations, establish the relationship between UT1 and UTC.
- Monitor the difference. IERS tracks UT1−UTC against the current bound of 0.9 seconds.
- Announce a decision. If an adjustment is needed, IERS publishes the decision in Bulletin C, with the applicable date and direction.
- Apply the UTC adjustment. Time services incorporate the announced second at the stated month boundary; the SI second itself remains unchanged.
Bulletin C is therefore the place to look for the formal decision, rather than relying on a presumed annual pattern or an old schedule. The at-least-eight-weeks notice described in the IERS document is an announcement lead time. It should not be mistaken for a guarantee that a leap second will be announced on a fixed date every year.
What the latest announcement says
IERS Bulletin C 72, issued in Paris on 6 July 2026, states: “NO leap second will be introduced at the end of December 2026.” The same bulletin states that, from 1 January 2017 at 0h UTC until further notice, UTC−TAI = −37 seconds.
These are operational statements with specific scopes, not a permanent promise about future leap seconds. The December 2026 notice addresses that possible date; “until further notice” qualifies the stated UTC−TAI offset. Because IERS decisions can change, the current Bulletin C is the authority to consult for a later date.
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The two statements also answer different questions. The no-leap-second notice identifies the decision for the end of December 2026. The UTC−TAI figure describes the offset stated as continuing from 1 January 2017 until further notice. Neither should be turned into a claim that there can never be another adjustment; later IERS announcements may update the status.
What leap seconds do—and do not—mean
- They do keep two kinds of time aligned within a limit. UTC stays close to the rotation-based UT1, with the absolute difference held below 0.9 seconds under the current system.
- They do not make every day longer or shorter. The adjustment occurs only when announced, not as a routine daily correction.
- They do not change the SI second. The correction affects UTC’s count at a boundary; it is not a redefinition of the atomic unit of time.
- They do not follow a dependable timetable. Earth’s rotation varies, and the need for an adjustment is determined by observation.
- They do not make UT1, UTC, TAI and TT synonymous. Each scale has a distinct basis or use, so calculations should follow the time scale specified by the relevant data or model.
Why this distinction matters
For ordinary civil timekeeping, UTC provides a coordinated standard while maintaining a link to the solar day. For astronomy, geodesy, and work involving Earth’s orientation, UT1 carries information about the planet’s actual rotation. TAI and TT serve atomic and computational roles that should not be collapsed into a civil-clock label.
The practical lesson is to identify the time scale before interpreting a timestamp. A value stated in UTC answers a civil-time question; a UT1 value conveys Earth-rotation time; TAI and TT may appear in atomic-time records or astronomical calculations. When precise work depends on Earth orientation, the current IERS products and announcements determine the applicable relationship.
For a reader asking why leap seconds are needed, the short answer is that Earth’s rotation and atomic time provide different kinds of stability. Atomic seconds stay uniform, but the rotation-based day does not always contain exactly 86,400 of them. UT1 records the rotation, UTC serves as civil time, and the announced adjustment keeps the two close without changing the SI second.
Frequently Asked Questions
How does Earth’s rotation affect timekeeping?
Earth’s rotation is variable, so a rotation-based day is not always exactly 86,400 SI seconds. Atomic clocks provide uniform seconds; UT1 is derived from observed rotation, while UTC is adjusted occasionally to remain close to UT1.
What is the difference between UT1 and UTC?
UT1 is Earth-rotation time, derived from observations and used to represent Earth’s orientation. UTC is the atomic-time-based civil scale, adjusted with leap seconds so that the absolute difference |UT1−UTC| stays below 0.9 seconds under the current system.
Why do we need leap seconds?
They keep civil UTC close to Earth-rotation time UT1 even though Earth’s rotation varies and atomic seconds remain uniform. The adjustment changes UTC’s sequence at a boundary; it does not change the length of the SI second.
How often are leap seconds added?
There is no fixed or predictable schedule. IERS monitors Earth-rotation measurements and announces whether an adjustment is needed; the current announcement format uses the end of June or December.
Who decides whether to introduce a leap second?
The IERS decides and announces leap-second decisions through Bulletin C. Its description reproduces an ITU recommendation that an announcement should be made at least eight weeks in advance.
Will there be a leap second at the end of December 2026?
IERS Bulletin C 72, issued on 6 July 2026, says no leap second will be introduced at the end of December 2026. That notice applies to that date, not permanently to all future dates.
What do TAI and TT have to do with UTC?
TAI is International Atomic Time; IERS Bulletin C 72 gives UTC−TAI = −37 seconds from 1 January 2017 until further notice. TT is used as a time argument in some astronomical models and has the conventional relation TT = TAI + 32.184 seconds. They have distinct uses and should not be treated as interchangeable with UTC or UT1.
The Bottom Line
Earth’s rotation varies, but atomic seconds remain uniform. UT1 tracks rotation, UTC is the adjusted civil scale, and IERS uses observed Earth-orientation data to decide whether a leap second is needed. The adjustment keeps |UT1−UTC| below 0.9 seconds under the current system without changing the SI second. IERS Bulletin C 72 says there will be no leap second at the end of December 2026.
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