Today it feels obvious that the world is divided into time zones and that noon in London is 7 a.m. in New York. But this system is barely 140 years old, and it was created to solve a very practical problem: trains.
For most of history, noon meant the moment the sun was highest in the sky. That is a local event. When it's noon in one town, it's a few minutes earlier or later in a town a few dozen miles east or west. For centuries this didn't matter, because nobody travelled fast enough to notice.
Railways changed that. In the 1840s British railway companies began running on a single "railway time" based on Greenwich, so timetables made sense. Before that, a station could have a clock for local time and another for the railway. Britain later made Greenwich Mean Time the legal standard with the Statutes (Definition of Time) Act 1880. You can read more on the London page.
The problem was worse in North America, where railways served a whole continent. Before 1883 there were dozens of local times in use. On 18 November 1883 the railroads adopted four standard time zones across the continent. Some cities reset their clocks at noon, and in places the clocks were turned back, which led newspapers to call it the "day of two noons". The US federal government formalised the system much later, with the Standard Time Act of 1918. See how it plays out today in New York, Chicago and Los Angeles.
In 1884 delegates from around 25 nations met in Washington, D.C., for the International Meridian Conference. They chose the meridian passing through the Royal Observatory at Greenwich as the world's prime meridian, the zero line for longitude and time. That decision underpins the world's time scale, now known as UTC.
The conference recommended a universal day but did not create time zones for the world. Countries adopted zones on their own schedules. Japan chose a single zone, UTC+9, in 1888 (see Tokyo). Many others followed over the next decades.
Moving the clocks in summer was an idea proposed by a few people before the First World War, but it became real in 1916, when Germany and its allies introduced it to save fuel in wartime. The United Kingdom followed weeks later with British Summer Time, and many other countries did soon after. Some dropped it after the war and adopted it again later, and rules have been changed many times since.
Today roughly a third of the world's countries use daylight saving in some form. Others abandoned it, and the rules for those that still use it differ. That is why the time difference between two cities can change twice a year.
Time zones follow politics as much as geography:
Governments still change their clocks. They change daylight saving dates, abolish it or introduce it, and occasionally shift a whole country by an hour. Software must keep up. The world's time zone database, maintained by volunteers, is updated several times a year because of these decisions.
Each of our city pages includes its current UTC offset and a short note on its time zone history. If you plan a call across zones, the world clocks let you compare several places at a glance.
To learn how the time scale underneath all this is produced, read how atomic clocks work.