GMT: Greenwich Mean Time
GMT (Greenwich Mean Time) is a time zone based on the mean solar time at the Royal Observatory in Greenwich, London. It was the world's time standard from 1884 until 1972. GMT is still used as a time zone designation in the UK (outside British Summer Time) and in parts of West Africa. When you hear "London time in winter," that is GMT.
UTC: Coordinated Universal Time
UTC (Coordinated Universal Time) replaced GMT as the world's primary time standard in 1972. Unlike GMT (which is based on Earth's rotation), UTC is based on atomic clocks, making it far more precise. UTC is used by the internet, aviation, military, and all modern computing systems. When a server timestamp says "2025-04-01T14:00:00Z", that Z means UTC.
The Practical Difference
For everyday time conversion purposes, GMT and UTC are the same — both are UTC+0. The difference only matters at a sub-second precision level due to leap seconds, which UTC uses to stay synchronized with Earth's rotation. For scheduling meetings, travelling, or building apps, treat GMT = UTC.
When Should You Use Each?
Use UTC when: writing code, logging events, database timestamps, API communication, or any technical context. Use GMT when: discussing time zones conversationally, especially for UK-based schedules. Both refer to the same clock reading. The choice is about convention, not accuracy.