What Is GMT — Greenwich Mean Time

GMT, or Greenwich Mean Time, is the mean solar time calculated from the prime meridian, which passes through the previous location of the transit instrument at the Royal Observatory in Greenwich, England, just outside of London.

Today, GMT is not the universally accepted official time reference point. It was replaced by Universal Time Coordinated, or UTC. Nevertheless, the abbreviation GMT is still used to signify time offset in certain smartphones and operation systems (in time zone settings), as well as on websites and in some places.

Where is GMT used?

For everyday people and their daily lives, there's no difference between GMT and UTC. In fact, it's widely accepted that Greenwich Mean Time is equal to within one second of Universal Time Coordinated. That's why GMT and UTC mean the same thing on telephones, computers, and wristwatches (both mechanical and electronic). For example, one device might display time zones as "GMT+1/2/3/4…", while another shows them as "UTC+1/2/3/4…". In both cases, gadgets will display the same time.

How GMT came to be

In 1847, Greenwich Mean Time was adopted by the Railway Clearing House of Great Britain, a country that played a crucial role in creating this system for referencing time. However, it wasn't until 1880 that this new system for measuring time was made the law of the land.

In 1851, the decision was made to locate the prime meridian at the Greenwich Observatory.

In 1884, the International Meridian Conference was held in Washington, DC, where voting results selected the Greenwich Meridian to be the prime meridian. Ever since then, this prime meridian has been used in geography and other related sciences by all countries in the world.

Why the World Is Turning Away from GMT

Greenwich time is determined by astronomical observations of the daily movement of stars. In other words, it's tied to the Earth's rotation. And while the Earth's rotation is not constant, it continues to slow, something which the Greenwich measurement system does not take into account. GMT also fails to account for the movement of stars and planets in relation to one another. Every 500 days, Greenwich time loses 1 second relative to the actual time. This difference is critical in modern-day scientific applications.

The invention of atomic clocks – and their unearthing of inaccuracies in the Greenwich timekeeping system – served as the primary reason why the international community began to turn away from GMT.

The Universal Time Coordinated system in use today varies only slightly in determining the accurate time, 1 second in 30 million years.