UTC vs. GMT: what’s the difference?

By Adam Gouda · Last reviewed November 9, 2026

In everyday use, UTC and GMT show the same time, and many people use the terms interchangeably. But they come from different ideas: one is based on atomic clocks, the other on the Sun over Greenwich.

GMT: time from the Sun

Greenwich Mean Time is based on the average (“mean”) position of the Sun as seen from the Royal Observatory in Greenwich, London, which sits on the prime meridian, 0° longitude. In 1884, the International Meridian Conference adopted Greenwich as the world’s prime meridian. For much of the 20th century, GMT was the world’s reference time.

Today, GMT is also the name of a time zone. The UK uses GMT (UTC+0) in winter and switches to British Summer Time (BST, UTC+1) in summer.

UTC: time from atomic clocks

Coordinated Universal Time is the modern world time standard. It’s built from International Atomic Time (TAI), a weighted average of hundreds of atomic clocks in laboratories around the world, coordinated by the International Bureau of Weights and Measures (BIPM) near Paris. Atomic clocks are far steadier than Earth’s rotation, which speeds up and slows down very slightly.

UTC is a standard, not a time zone. It never changes for daylight saving time. It’s the “Zulu” time used in aviation (see what is Zulu time?).

Leap seconds: keeping the two in step

Because Earth’s rotation is slightly irregular, astronomical time (UT1) and atomic time drift apart. Since 1972, UTC has been kept within 0.9 seconds of Earth-rotation time by occasionally inserting a leap second, usually at the end of June or December. In total, 27 leap seconds have been added, the most recent on December 31, 2016. UTC is now 37 seconds behind pure atomic time.

Leap seconds cause real problems for computers, networks and financial systems, which aren’t designed for a minute with 61 seconds. Recently, Earth’s rotation has sped up, raising the prospect of the first-ever negative leap second, which would remove a second. Many systems have never been tested for that.

The end of the leap second

In 2022, the General Conference on Weights and Measures (CGPM) resolved to raise the allowed gap between UTC and Earth-rotation time by 2035. On October 15, 2026, at its 28th meeting in Versailles, the CGPM adopted a resolution under which UTC will run without leap seconds from May 20, 2027, allowing the gap to grow to as much as one hour before any correction is considered. In practice, the leap second of December 31, 2016 is expected to be the last.

For everyday clocks, including this one, nothing visible changes. Your device keeps showing UTC and local time exactly as before.

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