UTC is the primary time standard by which the world regulates clocks and time. It is a highly precise atomic time standard that is kept in close alignment with the Earth's rotation. All time zones are expressed as an offset from UTC.
How UTC differs from GMT
Greenwich Mean Time (GMT) was the original international time standard, based on the mean solar time at the Royal Observatory in Greenwich, London. UTC replaced GMT as the scientific standard in 1972. While GMT is based on astronomical observation, UTC is based on atomic clocks — specifically, a weighted average of over 400 atomic clocks in over 50 national laboratories worldwide. For civil purposes, UTC and GMT are effectively identical (they never differ by more than 0.9 seconds), but UTC is the term used in science, aviation, computing, and telecommunications.
How UTC is maintained
UTC is maintained by the Bureau International des Poids et Mesures (BIPM) in Paris, which combines data from atomic clocks worldwide to produce a weighted average called TAI (International Atomic Time). UTC stays within 0.9 seconds of Earth's rotation by adding or removing leap seconds — though no leap seconds have been added since December 2016, and there is an ongoing discussion about abolishing them entirely.
Why UTC matters for computing
Every modern operating system, database, and web application stores timestamps in UTC. When you see a local time on your phone or computer, it's actually UTC with your timezone offset applied. This is why servers across different countries can coordinate events, log entries, and transactions with sub-second precision — they all agree on the same reference time. Without UTC, international banking, air traffic control, stock exchanges, and the internet itself would be impossible to coordinate.
Common UTC offsets
UTC+0 is the baseline (same as GMT). Common offsets include UTC-5 (US Eastern Standard Time), UTC-8 (US Pacific Standard Time), UTC+1 (Central European Time), UTC+5:30 (India Standard Time), UTC+8 (China Standard Time), and UTC+9 (Japan Standard Time). Some zones use 30- or 45-minute offsets — for example, Nepal is UTC+5:45, and the Chatham Islands are UTC+12:45.
For a more detailed exploration, please read our full in-depth article on UTC vs GMT.