Greenwich clock with standard measurements
The vertical red line left of the middle is the Greenwich meridian.
Clock in Kumasi, Ghana, set to GMT.
Local times in major towns across the United States, as published in 1857 when, as the document states, 'There is no "Standard Railroad Time" in the United States or Canada'.

In Britain, the Great Western Railway had standardised time by 1840 and in 1847 the Railway Clearing Union decreed that "GMT be adopted at all stations as soon as the General Post Office permitted it".

- International Meridian Conference

As the United Kingdom developed into an advanced maritime nation, British mariners kept at least one chronometer on GMT to calculate their longitude from the Greenwich meridian, which was considered to have longitude zero degrees, by a convention adopted in the International Meridian Conference of 1884.

- Greenwich Mean Time
Greenwich clock with standard measurements

2 related topics with Alpha

Overall

Standard time zones of the world (February 2021). The number at the bottom of each zone specifies the number of hours to add to UTC to convert it to the local time.

Universal Time

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ERA = 2π(0.7790572732640 + 1.00273781191135448Tu) radians

ERA = 2π(0.7790572732640 + 1.00273781191135448Tu) radians

Standard time zones of the world (February 2021). The number at the bottom of each zone specifies the number of hours to add to UTC to convert it to the local time.
An 1853 "Universal Dial Plate" showing the relative times of "all nations" before the adoption of universal time

Starting in 1847, Britain established Greenwich Mean Time, the mean solar time on the Prime Meridian at Greenwich, England, to solve this problem: all clocks in Britain were set to this time regardless of local solar noon.

The development of Universal Time began at the International Meridian Conference.

Time zones of the world

Time zone

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Area that observes a uniform standard time for legal, commercial and social purposes.

Area that observes a uniform standard time for legal, commercial and social purposes.

Time zones of the world
Time zones of the world
Plaque commemorating the Railway General Time Convention of 1883 in North America
The control panel of the Time Zone Clock in front of Coventry Transport Museum
1913 time zone map of the United States, showing boundaries very different from today
World map of time zones in 1928
Difference between sun time and clock time during daylight saving time:

The Royal Observatory, Greenwich, founded in 1675, established Greenwich Mean Time (GMT), the mean solar time at that location, as an aid to mariners to determine longitude at sea, providing a standard reference time while each location in England kept a different time.

He advocated his system at several international conferences, including the International Meridian Conference, where it received some consideration.