GMT (Greenwich Mean Time) is the clock time at the Royal Observatory in Greenwich, London. It has been used as a reference for timekeeping since 1884.
For most practical purposes, GMT and UTC show the same time. The key difference is that GMT is defined by the Earth's rotation, while UTC is defined by atomic clocks. Read the full comparison in UTC vs GMT.
Greenwich Mean Time is the mean solar time at the Royal Observatory in Greenwich, England, located on the Prime Meridian (0° longitude). It was established in 1675 and became the international standard for civil timekeeping at the International Meridian Conference in 1884.
Before the adoption of time zones, each city set its clocks to local solar noon. As railways and telegraphs connected distant cities, a single reference time became necessary. GMT served that role for nearly a century before being largely replaced by UTC in 1960.
Several countries use GMT (UTC+0) as their standard time, either year-round or outside of daylight saving time:
While GMT and UTC typically display the same time, they are technically different:
For everyday purposes, the difference is negligible (always less than 1 second). However, scientific and technical applications always use UTC because of its superior accuracy.
Read the full explanation: UTC vs GMT — What's the Difference?