UTC (Coordinated Universal Time)
The world's common time standard. Each time zone is expressed as an offset from UTC.
UTC (Coordinated Universal Time) is the standard time used as the international reference for time. It is calculated from a weighted average of many atomic clocks installed around the world and is managed by the International Bureau of Weights and Measures (BIPM). A time zone is expressed as an offset (difference) from UTC. Japan Standard Time (JST) is UTC+9.
The technical background of UTC
Why UTC, and not CUT or TUC?
In English it would be Coordinated Universal Time (CUT), and in French Temps Universel Coordonné (TUC), so UTC was adopted as a compromise that favors neither language. It is not an abbreviation but an international symbol that does not depend on any language.
Leap seconds and UTC
Because the Earth’s rotation speed is not constant, a gap arises between UTC, which is based on atomic clocks, and astronomical time (UT1), which is based on the Earth’s rotation. To keep this gap from exceeding 0.9 seconds, “leap seconds” are inserted as needed. Since the system began in 1972, a total of 27 leap seconds had been inserted as of January 1, 2017. However, the 2022 General Conference on Weights and Measures adopted a resolution to raise the upper limit on the difference between UT1 and UTC by 2035.
UTC in scheduling systems
Some scheduling tools are designed to store date and time data internally in UTC and convert it to the user’s local time zone when it is displayed. This makes it easier to keep times consistent across daylight saving time changeovers and among participants in different time zones.
In ISO 8601 format, a UTC date and time is written with a Z at the end (for example, 2026-05-20T10:00:00Z).
How it differs from GMT
UTC is often confused with GMT (Greenwich Mean Time). In everyday contexts the two refer to almost the same time, but GMT is a historical reference based on astronomical observation (the meridian of the Royal Observatory, Greenwich), whereas UTC is a scientific definition based on atomic clocks, so strictly speaking they are different things. GMT survives as, among other things, the name of winter time in the United Kingdom. When indicating a reference time in international scheduling, choosing UTC notation reduces ambiguity.