Coordinated Universal Time
Coordinated Universal Time, or UTC, is the international reference time scale used to synchronize civil time, networks, navigation, science, and time zones around the world.
What UTC is
Coordinated Universal Time is the main reference scale for civil time. It gives the world a shared clock standard so that time stamps, broadcasts, navigation systems, financial records, weather data, and computer networks can agree on when something happened.
How UTC is coordinated
UTC is not read from a single clock. National laboratories and observatories maintain local realizations of UTC, such as UTC(NIST), and send clock data to the International Bureau of Weights and Measures. The BIPM uses those data to compute the international time scale and publish comparisons between UTC and each participating laboratory's realization.
Atomic time and Earth rotation
Modern UTC is based on International Atomic Time, which is built from extremely stable atomic clocks. It also stays close to Earth's rotation by using leap seconds when needed. That compromise lets UTC serve both precise technology and ordinary civil time, which is historically tied to the apparent motion of the Sun.
Leap seconds
A leap second is a one-second adjustment added to UTC to keep it close to UT1, a time scale based on Earth's rotation. Leap seconds are announced in advance by the International Earth Rotation and Reference Systems Service. They are rare, but they matter to systems that require exact time handling.
UTC and time zones
UTC itself is not a local time zone for most people. Local civil time is usually written as an offset from UTC. Bangkok is UTC+07:00, New York shifts between offsets depending on standard time and daylight saving time, and some regions use half-hour or quarter-hour offsets.
UTC, GMT, and Zulu time
Greenwich Mean Time is an older term connected to mean solar time at Greenwich and is still used in some legal and everyday contexts. UTC is the modern technical reference. In aviation, weather, and military communication, the letter Z or the phrase Zulu time often marks a UTC time stamp.
Why computers care
UTC gives software a neutral reference for storing and comparing times across time zones. A server can log an event in UTC, then convert it for users in Bangkok, London, or Los Angeles. This avoids many errors, though software still needs time-zone databases for daylight saving rules and historical local times.
Why it matters
UTC is quiet infrastructure. It supports satellite navigation, internet security, stock trades, radio signals, scientific measurements, emergency alerts, distributed databases, and everyday calendars. When systems share UTC, they can coordinate across distance without first agreeing on a local clock.