UTC (Coordinated Universal Time)
utc
The primary time standard by which the world regulates clocks and time, serving as the basis for civil timekeeping globally.
GMT (Greenwich Mean Time)
gmt
Overview
Greenwich Mean Time (GMT) is the mean solar time observed at the Royal Observatory in Greenwich, London, located on the Prime Meridian (0° longitude). Established in 1884 at the International Meridian Conference, GMT served as the world's time reference standard for over a century before being superseded by UTC in 1972.
GMT vs. UTC
While GMT and UTC are often used interchangeably in casual contexts, they differ fundamentally. GMT is defined by astronomical observation of the Sun, making it slightly irregular. UTC is defined by atomic clocks and adjusted with leap seconds. For most practical purposes the two are identical, but in scientific and legal contexts the distinction matters.
Modern Usage
Several countries in Western Africa and Western Europe still use GMT (UTC+0) as their civil time. The United Kingdom uses GMT in winter and switches to BST (British Summer Time, UTC+1) during summer months. In programming, the abbreviation "GMT" persists in many APIs and date formatting functions for historical compatibility.
Was this article helpful?
Related Terms
Related Articles
The History of Standard Time - How Railways Forced a Global Clock
Before railways, every town kept its own solar time. Trains made that impossible, and within a few decades the world adopted a coordinated system anchored to the Greenwich meridian. This article traces the journey from local solar time through the 1884 International Meridian Conference to UTC.
How Time Zones Work - Understanding UTC Offsets and the Global Time System
Learn how time zones divide the world into regions with different local times, how UTC offsets work, and why some zones use half-hour increments.
Daylight Saving Time - How Clock Changes Affect Global Coordination
Understand how daylight saving time works, which countries observe it, when transitions occur, and how DST complicates international scheduling.