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What is UTC? How world time is made

Published 28 October 2026

Hundreds of clocks, one time scale

No single clock keeps world time. Around 80 time laboratories send the BIPM data from about 450 atomic clocks. The Bureau combines them into one stable scale and publishes the results every month in a bulletin called Circular T.

Each national laboratory keeps its own real-time version of UTC, written UTC(k), where k stands for the laboratory. Circular T tells each of them how far its version was from UTC, so that they can steer their clocks. Time in a country is therefore traceable to the calculation made in Sèvres.

UTC and TAI

Behind UTC stands International Atomic Time, TAI. The two run at exactly the same rate and differ only by a whole number of seconds. The difference comes from leap seconds.

Why leap seconds

Atomic clocks are steadier than the Earth. Its rotation, the basis of the older time scale UT1, varies slightly. To keep UTC within 0.9 s of UT1, a leap second has been inserted when needed, following rules of the International Telecommunication Union.

Leap seconds have become a problem. A sudden extra second can cause serious faults in satellite navigation, telecommunications and energy networks, and operators handle it in different, uncoordinated ways. Recent data on the Earth’s rotation also suggest that a first ever negative leap second might be needed, which has never been tested.

The end of leap seconds

In 2022 the General Conference on Weights and Measures decided that the allowed difference between UT1 and UTC would be increased. At its next meeting, in October 2026, it set the details: from 20 May 2027 UTC becomes continuous, and the difference between UT1 and UTC may grow to as much as 3600 seconds, one hour. That margin is meant to keep UTC free of jumps for several centuries.

The BIPM keeps watching how far UT1 and UTC drift apart and works with the International Telecommunication Union, the International Earth Rotation and Reference Systems Service and other bodies to help users through the change.

The second itself has not changed yet. Optical clocks are now much better than the caesium clocks that define it, and in 2026 the General Conference asked for a proposal for a new definition to be presented at its next meeting, in 2030; see the Base units page.