Greenwich Mean Time (GMT) and Coordinated Universal Time (UTC) are often used as if they mean the same thing. For most practical purposes, they do — both represent the time at the prime meridian (0 degrees longitude) with no offset. But technically, they are defined differently and serve different purposes.
What is GMT?
Greenwich Mean Time is the mean solar time at the Royal Observatory in Greenwich, London. It was established in 1675 when the observatory was founded, and it served as the world's time standard from 1884 (after the International Meridian Conference) until 1960. GMT is based on astronomical observation — specifically, the average position of the sun as seen from Greenwich. Because the Earth's rotation is slightly irregular, GMT can drift by up to 0.9 seconds from atomic time.
What is UTC?
Coordinated Universal Time is the modern time standard, maintained since 1960 by the International Bureau of Weights and Measures. Unlike GMT, UTC is based on atomic clocks rather than astronomical observation. It is calculated from a network of over 400 cesium and hydrogen maser clocks worldwide. UTC is kept within 0.9 seconds of mean solar time through the occasional insertion of leap seconds.
Key differences
The fundamental difference is precision and definition. GMT is defined by the Earth's rotation relative to the sun — an astronomical measurement. UTC is defined by atomic clocks — a physical measurement. GMT can vary slightly because Earth's rotation is not perfectly constant. UTC is precise to the nanosecond. In practice, GMT and UTC never differ by more than 0.9 seconds, which is why they are treated as identical for everyday purposes like scheduling meetings or setting clocks.
When does the distinction matter?
For everyday use — converting meeting times, checking world clocks, planning travel — GMT and UTC are interchangeable. The distinction matters in scientific and technical contexts: satellite navigation (GPS uses its own time scale related to UTC), astronomical observation, financial trading (where milliseconds matter), network time synchronization (NTP uses UTC), and legal definitions of time. The United Kingdom legally uses GMT as its standard time in winter, while aviation and most international standards use UTC.
Usage in modern contexts
Today, UTC is the preferred term in technical and international contexts. Programming languages, databases, and APIs use UTC. Aviation and the military use UTC (often called "Zulu time" and abbreviated with a Z). Scientific publications use UTC. However, GMT persists in common usage, especially in the UK and among the general public. Many timezone conversion tools (including ours) treat GMT and UTC as equivalent, which is correct for all practical purposes.
Bottom line
GMT and UTC represent the same time for all practical purposes. The difference is that GMT is an older standard based on solar observation, while UTC is the modern standard based on atomic clocks. When you see either term, you can treat them as meaning "the time at the prime meridian with no offset." For technical precision, prefer UTC.