UTC (Coordinated Universal Time) and GMT (Greenwich Mean Time) usually display the same time. But they are defined differently, and in technical contexts the distinction matters.
| UTC | GMT | |
|---|---|---|
| Based on | Atomic clocks (International Atomic Time) | Earth's rotation (mean solar time at Greenwich) |
| Established | 1960, officially adopted 1967 | 1884 (International Meridian Conference) |
| Type | Time standard (not a time zone) | Originally a time standard; now used as a time zone name (UTC+0) |
| Accuracy | Extremely precise (atomic precision) | Less precise (depends on Earth's variable rotation) |
| DST | Never changes | Never changes |
| Used by | Aviation, computing, science, international standards | Some countries as civil time (UK in winter, Iceland, Ghana) |
UTC is calculated from a weighted average of over 400 atomic clocks worldwide, coordinated by the International Bureau of Weights and Measures (BIPM). It is adjusted with occasional leap seconds to stay within 0.9 seconds of the Earth's actual rotation.
GMT is defined by the position of the sun as observed from the Royal Observatory in Greenwich, London. Because the Earth's rotation speed varies slightly, GMT is inherently less precise than UTC.
For everyday purposes, the difference between UTC and GMT is always less than one second and is irrelevant. For scientific, computing, or aviation applications, UTC is always preferred.
The abbreviation is a compromise. English speakers proposed "CUT" (Coordinated Universal Time), while French speakers preferred "TUC" (Temps Universel Coordonné). The International Telecommunication Union settled on "UTC" as a language-neutral abbreviation that doesn't exactly match either language.