Greenwich Mean Time (GMT) is the world’s first standardized time system and still serves as the global reference for timekeeping. This guide covers what GMT is, how it came to be, where it’s used, and how it relates to modern time zones.

Table of Contents

What Is GMT?

Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London. It is considered the zero point for all global time zones.

GMT vs. UTC

While technically defined differently, GMT and Coordinated Universal Time (UTC) are functionally equivalent. Both are used as UTC±00:00 and are often used interchangeably in everyday and scientific settings.

GMT Origins & Prime Meridian

GMT was chosen as the global standard for several reasons:

  • The Royal Observatory in Greenwich was the site of the Prime Meridian (0° longitude).
  • It served as a logical and scientific center for defining global time standards starting in the mid-1800s.
  • GMT was officially adopted in the UK in 1847 to help synchronize railway schedules.
  • Other countries followed over the decades—Ireland in 1916, France in 1911, and Spain until 1940.

Countries & Cities That Use GMT

Winter Observers of GMT

  • United Kingdom
  • Republic of Ireland
  • Portugal
  • Morocco
  • Western Sahara

These regions switch to UTC+1 (e.g., BST or WEST) during the summer months due to daylight saving time.

Year-Round Observers of GMT

  • Iceland
  • Ghana
  • Ivory Coast
  • Guinea
  • Burkina Faso
  • Mali
  • Senegal
  • Togo
  • Guinea-Bissau
  • Liberia
  • Sierra Leone
  • Faroe Islands
  • Canary Islands (Spain)
  • Saint Helena, Ascension, and Tristan da Cunha (UK Territory)

Summer-Only Observers of GMT

  • Azores (Portugal) – shifts from UTC-1 to GMT (UTC±0) in summer
  • Eastern Greenland (e.g., Ittoqqortoormiit)

Major Cities in the GMT Zone

  • London – GMT’s birthplace and largest city in the zone
  • Dublin – Capital of Ireland
  • Lisbon – Capital of Portugal
  • Casablanca – Largest city in Morocco

Maritime & Navigational Use

Historically, British mariners used GMT to calculate their ship’s longitude using a marine chronometer set to GMT. Comparing this to local solar noon helped determine east-west position—critical in global navigation.

GMT and Daylight Saving Time

Greenwich itself doesn’t use GMT year-round. From the last Sunday in March through the last Sunday in October, the UK follows British Summer Time (BST), which is GMT+1. This system was established with the Summer Time Act of 1916.

Geographic Range of GMT

  • Westernmost point using GMT: The smallest of Spain’s Canary Islands at around 18°00' W
  • Easternmost point using GMT: Lowestoft, East Anglia, in the United Kingdom

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