What is Coordinated Universal Time (UTC)?

Is UTC a time zone?

UTC is not itself a time zone. It is the reference clock against which every time zone is defined, sometimes adjusted seasonally by daylight saving rules. London local time, for example, is UTC in winter and UTC+1 in summer; UTC itself does not shift with the seasons. The IANA Time Zone Database — the operational source of truth for time-zone rules, used by every major operating system and programming language — encodes each zone as a sequence of UTC offsets and daylight-saving rules.²

UTC formally replaced Greenwich Mean Time (GMT) as the international standard on 1 January 1972, when the modern leap-second system began.⁵ The two names are still used interchangeably in casual writing, and for most practical purposes they refer to the same instant. In technical usage GMT now means the time zone at the prime meridian — a local label of UTC+0 — rather than a separate astronomical scale.⁷

How does UTC stay aligned with the Earth's rotation?

The Earth's rotation is slightly irregular. Tides slow it gradually over centuries; the seasonal redistribution of air and water across the planet speeds it up and slows it down by milliseconds within a single year; large earthquakes nudge it by smaller amounts still. Atomic clocks, by contrast, do not care about any of this. They tick at a fixed rate — the international second, defined since 1967 by the resonant frequency of caesium atoms — and they tick at that rate forever.

UTC reconciles the two by ticking at the atomic-clock rate while inserting an extra second every so often to keep the cumulative drift bounded. The agreed limit, set by an international standard for time-signal broadcasts, is that UTC must stay within 0.9 seconds of UT1, the time scale defined by the Earth's actual rotation.¹ When the gap is about to break that bound, the body responsible — the International Earth Rotation and Reference Systems Service, or IERS — announces a one-second insertion to take effect at the end of either 30 June or 31 December.¹⁸ That extra second is the leap second.

Each leap second adds one to the integer offset between UTC and the underlying continuous atomic-time scale, called International Atomic Time (TAI). The ticks themselves never change — only the count of how many extra seconds have been inserted along the way.¹

How many leap seconds have been added?

Twenty-seven leap seconds have been added to UTC between 1972 and 2017. UTC therefore currently runs 37 seconds behind International Atomic Time. That 37 is the sum of two parts: a 10-second seed value applied at the start of 1972 — which carried forward the drift that had accumulated between atomic clocks and the Earth's rotation since the atomic-time scale itself was anchored in 1958 — and the 27 leap-second insertions since.⁵³ The first true leap second was inserted at the end of 30 June 1972.⁸

The most recent leap second was added at the end of 2016. Bulletin C 71 — the latest issue of the IERS bulletin that announces upcoming insertions — confirms that no leap second will be inserted at the end of June 2026.⁹

Where does the name "Coordinated Universal Time" come from?

The name predates the modern leap-second system by twelve years. From January 1960, the major national observatories — the Royal Greenwich Observatory in Britain, the U.S. Naval Observatory, and the National Physical Laboratory in Teddington — began coordinating their broadcast time signals: adjusting the rates of their atomic clocks and applying small step corrections so they would all stay close to the Earth's rotation. The result was called, informally at first, Coordinated Universal Time.⁵ The Bureau International de l'Heure took over international coordination in 1961, and the international body responsible for radio time signals formalised the system as a recommendation in 1963.⁵

The same body settled the official names in 1967: Coordinated Universal Time in English, Temps Universel Coordonné in French, with the acronym UTC used in both languages — a compromise that matched neither expansion exactly but that everyone could live with.⁵

The early UTC was awkward in practice. Every year a new frequency offset was set so broadcast clocks would run slightly off the atomic-clock rate to track the Earth's rotation; when the prediction failed, step adjustments of 50 ms (later 100 ms) were applied. Equipment had to retune annually, and by the late 1960s the proposed air-traffic collision-avoidance systems of the time could not tolerate the offsets at all. In May 1968 two metrologists — Gernot Winkler of the U.S. Naval Observatory and Louis Essen, who had built the first caesium atomic clock — independently proposed the modern scheme: keep the broadcast frequency fixed at the atomic-clock rate, and apply integer-second corrections instead. The international radio body adopted the proposal at its plenary in New Delhi in January 1970, and the new system began on 1 January 1972 after a final special offset of −0.107 758 0 seconds was applied to set the gap to exactly 10 seconds.⁵

Who maintains UTC?

Two international bodies do the bulk of the work.

The Bureau International des Poids et Mesures (BIPM), an intergovernmental measurement bureau outside Paris, computes UTC itself. It does so by averaging the readings of more than 300 atomic clocks at over 80 national time laboratories around the world to produce International Atomic Time, then deriving UTC from it by subtracting the integer leap-second offset.⁴¹⁰ Every month it publishes a short report listing how each laboratory's local clock differs from official UTC at five-day intervals.¹⁰ National laboratories, in turn, keep their own working realisations of UTC for use by industry and government — UTC(NIST) at the U.S. National Institute of Standards and Technology in Boulder, Colorado, and UTC(USNO) at the U.S. Naval Observatory in Washington, D.C., for example. They typically agree with each other and with the BIPM's official UTC to within about twenty nanoseconds.⁴

The work of measuring the Earth's actual rotation — and announcing leap seconds when the gap to atomic time is about to exceed the 0.9-second limit — falls to the IERS, introduced earlier. Its bulletins are the operational record of when leap seconds happen.⁹

What happens to UTC in 2035?

The leap second is on its way out. In November 2022 the global body that governs measurement units — the General Conference on Weights and Measures, which sets the definitions of the metre, the kilogram, and the second — passed a resolution to retire it. The maximum allowed gap between UTC and the Earth's rotation will be widened, in or before 2035, to a value chosen so that no further leap seconds will be needed for at least a century. At that point UTC will become a continuous atomic scale that drifts slowly away from rotational time over many decades, much as GPS time and TAI already do.⁶

The motivation is operational. Leap seconds are awkward in computer systems. Different cloud providers, operating systems, and time-server vendors have adopted different ad-hoc strategies for "smearing" a leap second over a window of hours, in different shapes, and the resulting inconsistencies have caused real outages in satellite navigation, telecom, and electrical grid systems.⁶ The push to retire the leap second has been a long one — discussion within the international metrology community began in 1999 — and the 2022 vote settled a debate over twenty years old.⁵ The remaining work is to choose the new tolerance value and write the implementation plan; that is expected to come before the next General Conference, which meets in November 2026.⁶

How does UTC compare with GMT, TAI, and GPS time?

Greenwich Mean Time (GMT) is now the time zone at the prime meridian — a local label, not a separate timescale. London local time is GMT in winter (which equals UTC) and BST (British Summer Time) in summer (UTC+1). For everyday purposes, GMT and UTC mean the same thing.⁷⁴

International Atomic Time (TAI) is the underlying continuous atomic timescale that UTC is built from. It has no leap seconds. The difference between TAI and UTC is exactly the integer count of leap seconds added since 1972 plus the original 10-second seed — currently 37 seconds.¹⁴

UT1 is the time scale defined by the Earth's actual rotation, computed from observed star transits with corrections for the wobble of the Earth's spin axis. UT1 drifts irregularly because the Earth itself does. The 0.9-second band between UTC and UT1 is the rule that triggers leap seconds.⁷

GPS time is a continuous atomic scale used by the GPS satellite system. It started equal to UTC at the beginning of 6 January 1980 and has not had a leap second since. It therefore leads UTC by the count of leap seconds added in the meantime — currently 18.⁴

How is UTC written in timestamps?

The standard way to write a UTC timestamp on the internet is in ISO 8601 form, with a Z suffix marking UTC. A timestamp such as 2026-05-02T14:30:00Z means 14:30:00 UTC on 2 May 2026, with no further interpretation needed. The Z is shorthand for an offset of +00:00; military and aviation tradition reads it as "Zulu", which is why UTC timestamps are sometimes called Zulu time.¹¹

The detailed rules for how internet timestamps are written come from a short specification published in 2002, with a 2024 extension that adds optional bracketed tags for time-zone names — for example 2026-05-02T07:30:00-07:00[America/Los_Angeles] — so that a timestamp can carry both an instant and the rule set that produced its local label.¹¹¹²

One detail worth knowing: the IANA Time Zone Database, which encodes every region's history of UTC offsets and daylight-saving rules and is used by every major operating system, does not encode leap seconds. Application code that needs to handle leap seconds correctly — typically scientific or astronomy software — uses TAI, GPS time, or a vendor-specific monotonic clock instead.²

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