How Time Zones Were Invented: A Short History
For most of history every town kept its own time, set by the sun. Time zones are a surprisingly recent invention, created because trains and telegraphs made differences between local times a practical problem.
Local solar time
Before the 1800s, noon was the moment the sun was highest in the sky at your location. Two towns a short distance apart east and west had slightly different clock times, but nobody traveled fast enough for it to matter.
Railways need a common time
Railways changed that. In Britain, railway companies began to use Greenwich Mean Time for their timetables in the 1840s, and by the 1850s most of the country kept railway time. In North America the problem was bigger, because a single trip could cross many local times. On 18 November 1883 US and Canadian railroads switched to a system of standard time zones, sometimes called the "day of two noons" because some towns saw noon twice. The Canadian engineer Sandford Fleming had been promoting the idea of worldwide time zones in the years before.
The Prime Meridian Conference of 1884
In 1884, delegates from many countries met in Washington, D.C., for the International Meridian Conference. They chose the meridian passing through Greenwich in London as the prime meridian, the zero line for longitude and for time. This is why Greenwich Mean Time became the world's reference. Countries then adopted standard time one by one: Japan, for example, introduced it in 1888.
The arrival of daylight saving time
Germany introduced daylight saving time in 1916 during the First World War to save fuel, and the United Kingdom followed within weeks. The United States first used it in 1918. Since then countries have adopted, dropped and changed it many times for energy, economic and political reasons.
From GMT to UTC
As atomic clocks became more accurate than the rotation of the Earth, the world moved to Coordinated Universal Time (UTC). The modern form of UTC, with occasional leap seconds, began in 1972. Leap seconds have not been added since the end of 2016, and in 2022 an international body voted to stop using them by 2035. For daily life UTC and GMT show the same clock time.
Why time zone rules keep changing
Time zones are decided by governments, so they change whenever laws change. Mexico ended daylight saving time in 2022, Egypt brought it back in 2023, and countries have moved their offsets by whole hours or half hours. To keep software correct, volunteers maintain the IANA time zone database, which is hosted by IANA and updated several times a year. Browsers and operating systems include a copy, and Time Atlas World reads the copy in your browser to calculate times.
Related: learn how to calculate differences in UTC and Time Zone Offsets, or read about unusual time zones.
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