UTC, Coordinated Universal Time, is the world's reference time scale. Every time zone is described by its offset from UTC: London is UTC+0 in winter and UTC+1 in summer, New York is UTC−5 or UTC−4, Tokyo is UTC+9 all year. UTC itself never changes for daylight saving time.
How UTC is kept
UTC is calculated by the International Bureau of Weights and Measures (BIPM) near Paris. The BIPM combines readings from hundreds of atomic clocks in laboratories around the world. This produces International Atomic Time (TAI), a perfectly even time scale. UTC follows TAI but has been kept within 0.9 seconds of the Earth's rotation by adding leap seconds. Since 1972, 27 leap seconds have been added, the last at the end of 31 December 2016, so UTC is now 37 seconds behind TAI. In 2022 the General Conference on Weights and Measures decided to allow the gap between UTC and the Earth's rotation to grow larger, by or before 2035, which will in effect end leap seconds.
Writing UTC times
- Offsets: local times are written with their offset from UTC, for example
2026-10-17T14:30+01:00for 14:30 BST in London. - The Z suffix: a trailing Z means UTC, as in
2026-10-17T13:30Z. Pilots and military users say "Zulu time". - Offsets are not time zones: "UTC+1" describes a single moment's offset. A time zone, such as
Europe/London, is the set of rules that decides which offset applies on each date. That is why software stores zone names rather than offsets.
UTC and GMT
For civil purposes, UTC and GMT give the same clock time. GMT is the older, astronomical reference, and UTC is the atomic standard that replaced it for science and technology. See GMT vs UTC for the history and the details.
Converting from UTC
To convert a UTC time to local time, add the place's offset for that date: 13:30 UTC is 14:30 in London in summer and 09:30 in New York in summer. Because the offsets change with daylight saving time, use the converter with the right date rather than a fixed number of hours.