The History of Standard Time

For most of human history, time was purely local. You knew what time it was by looking at the Sun, or by listening to the church bell that rang according to a sundial. Your neighbor a few miles east or west would have a slightly different noon. This was not a problem — until technology began moving people and information faster than the Sun moves across the sky.

Ancient and medieval timekeeping

The ancient Egyptians divided the day into 12 hours of daylight and 12 hours of darkness — which meant hours were longer in summer (more daylight to divide) and shorter in winter. The Romans used the same system. This “seasonal hour” or “temporal hour” remained the dominant timekeeping method for more than a thousand years.

The mechanical clock, which appeared in Europe in the late 13th century, made equal-length hours practical. Clock towers in cathedral cities became civic institutions — the tower in each city was set to local solar noon and governed the rhythm of life in that community. But each city’s clock was independent. There was no concept of a “national time” and no need for one.

The observatory and Greenwich Mean Time

The Royal Observatory at Greenwich was established in 1675 by King Charles II, with the practical goal of improving navigation. Ships at sea needed accurate tables of the Moon’s position to determine longitude. The Observatory became the world’s authority on astronomical time, publishing the Nautical Almanac from 1767 onward.

Greenwich Mean Time — the mean solar time at the Greenwich meridian — became the reference standard used by British and later most international shipping. By the mid-19th century, the vast majority of the world’s maritime charts used Greenwich as the prime meridian, giving it a de facto global status long before any international agreement.

The railways and the first standard time zones

The first practical standard time zone was created not by a government but by a railroad company. In 1840, the Great Western Railway of England adopted GMT as its timetable standard, distributed to every station via telegraph. Within a few years, most British railways followed. By 1855, 98% of British public clocks had been set to GMT.

In the United States, the situation was more chaotic. By the early 1880s, American railroads used 53 different local standard times. A traveler going from Maine to California would need to adjust their watch many times. On November 18, 1883, the major American and Canadian railway companies agreed to switch to four synchronized zones (Eastern, Central, Mountain, Pacific), each one hour apart based on Greenwich. This coordinated switch — executed via telegraph at noon on that day — is the origin of the North American time zone system still in use today.

Congress did not formally legalize this railroad time until the Standard Time Act of 1918 — 35 years later. In the intervening period, cities could and did maintain their own local times alongside railway time if they chose.

1884: The world chooses Greenwich

With standard time spreading across railroads and telegraphs worldwide, there was growing demand for a single global reference meridian. In October 1884, the United States hosted the International Meridian Conference in Washington, D.C., attended by delegates from 25 nations.

The vote to make Greenwich the prime meridian passed 22–1 (San Domingo voting against). France and Brazil abstained. France would not formally adopt Greenwich-based time until 1911, referring to its zone as “Paris Mean Time retarded by 9 minutes 21 seconds” to avoid acknowledging Greenwich explicitly.

The conference also recommended dividing the world into 24 hourly time zones centered on Greenwich, with half-zones possible for geographic convenience. This framework is still the basis of the modern timezone system.

Wartime timekeeping

Both World Wars produced significant timekeeping changes. Germany and Austria-Hungary introduced Daylight Saving Time in April 1916 to conserve fuel for the war effort. Britain and France followed within weeks. The US adopted DST in 1918, abandoned it after the war, then reinstated it in World War II as “War Time.”

During World War II, all of Britain ran on “Double Summer Time” in summer — two hours ahead of GMT — to maximize agricultural daylight. Occupied Europe ran on Berlin time regardless of geography, which is why France, Spain, and parts of Western Europe still use Central European Time (UTC+1) rather than the UTC+0 that their longitude would suggest.

The atomic clock and the birth of UTC

The 20th century brought a new problem: astronomical time — based on Earth’s rotation — was not precise enough for modern science and communications. Earth’s rotation is not perfectly uniform; it slows imperceptibly due to tidal friction and varies due to atmospheric pressure and the movement of tectonic plates.

The first atomic clock was built at the National Physical Laboratory in the UK in 1955. Atomic clocks measure time by the frequency of microwave radiation emitted by cesium-133 atoms — a rate so stable that the clock loses or gains less than one second per 300 million years. In 1967, the International System of Units redefined the second in terms of cesium atom vibrations, replacing the astronomical definition entirely.

In 1972, the world formally adopted Coordinated Universal Time (UTC) — maintained by a network of atomic clocks — as the international time standard. To keep UTC within 0.9 seconds of astronomical time (which is still used for navigation and some scientific purposes), the International Earth Rotation Service occasionally adds “leap seconds.” As of 2025, 27 leap seconds have been added since 1972.

Time today: a global synchronized network

Modern timekeeping is maintained by a global network of roughly 450 atomic clocks in laboratories across 80 countries. These are coordinated by the Bureau International des Poids et Mesures (BIPM) in Paris, which publishes International Atomic Time (TAI) and the UTC that derives from it.

Your smartphone synchronizes its clock to within milliseconds via GPS satellites (which carry their own atomic clocks) or via the Network Time Protocol (NTP), which traces back to the same global atomic clock network. When you check the time on your phone, you are effectively reading a clock derived from cesium atoms vibrating in laboratories around the world — and the whole chain from atom to display takes less than a second.

The political layer — who observes what offset, and whether DST applies — is a separate question, still managed by individual governments and recorded in the IANA timezone database. The physics are settled. The politics never are.