UTC time: universal time right now

What is UTC?

Coordinated Universal Time (UTC) is the reference time for the whole world. Every time zone is defined by its offset from it: New York is on UTC−5 in winter and UTC−4 in summer, Paris on UTC+1 in winter and UTC+2 in summer.

UTC does not belong to any country or season: it has no summer time. That is why aviation, weather services, sailors, radio amateurs and computer systems use it to date events without ambiguity.

It is derived from International Atomic Time (TAI), an average of several hundred atomic clocks in laboratories around the world, combined every month by the International Bureau of Weights and Measures near Paris. The abbreviation is a compromise between the English “Coordinated Universal Time” and the French “temps universel coordonné”: it matches the word order of neither language.

Leap seconds

The Earth’s rotation is irregular and very slowly slowing down, whereas atomic clocks do not drift. To keep UTC within 0.9 seconds of astronomical time, a leap second was added from time to time: the last minute of 30 June or 31 December then lasted 61 seconds, with a moment written 23:59:60.

Twenty-seven seconds were added between 1972 and 2016, the last one on 31 December 2016, so UTC is now 37 seconds behind International Atomic Time. Because these jumps cause trouble for computer networks and navigation systems, the General Conference on Weights and Measures decided in 2022 to abandon them by 2035 at the latest.

UTC and GMT: what is the difference?

GMT (Greenwich Mean Time) is the mean solar time at the meridian of the Royal Observatory in Greenwich, chosen as the prime meridian by the 1884 International Meridian Conference in Washington. It is an astronomical time, based on the Earth’s rotation; UTC is an atomic time, kept in step with it by leap seconds.

In everyday life the two are the same: the difference stays under one second. Note, however, that the United Kingdom is on GMT only in winter: from late March to late October it switches to British Summer Time (UTC+1).

Z, “Zulu” and ISO 8601

In aviation, at sea and in the military, UTC is written with the letter Z, pronounced “Zulu” in the radio alphabet: “1430Z” means 14:30 UTC. The ISO 8601 standard used in computing takes up the same letter: 2026-10-31T14:30:00Z is a moment in UTC, 2026-10-31T10:30:00-04:00 the same moment in New York in summer.