Time zones

Right now, it is not the same time everywhere.

When it is lunchtime in London, it is early morning in New York. People there are just waking up.

If you call a friend on the other side of the world, they might be eating breakfast while you are eating dinner. That is because clocks in different places show different times at the same moment.

For example, in winter, when it is noon in London, it is 7 in the morning in New York. NIST, the U.S. agency that keeps the nation's official time, explains that eastern standard time is five hours behind Coordinated Universal Time, the time at 0 degrees longitude near Greenwich in London. Each of these differences is set by a time zone.

A time zone is a region that shares one standard clock time. Encyclopaedia Britannica describes it as a zone on the globe approximately 15 degrees of longitude wide, extending from pole to pole, within which a uniform clock time is used. Every zone is defined by its offset from Coordinated Universal Time (UTC), the time of day at the prime meridian.

NIST gives the example of the U.S. East Coast: eastern standard time (EST) is UTC − 5. UTC itself does not change for daylight saving time, so eastern daylight time (EDT) is only four hours behind, UTC − 4. In military, nautical and aviation settings, NIST notes, UTC is still called 'Zulu' time, from the Z zone used for nautical time zones since the 1920s.

Earth spins all the time. Places take turns facing the Sun.

When the Sun is highest over you, it is noon. Places to the east had their noon earlier. Places to the west have theirs later.

Earth turns all the way around, 360 degrees, in about 24 hours. NOAA points out that this means it turns 15 degrees every hour. So local noon, when the Sun is highest in the sky, sweeps around the planet from east to west.

If every place set its clock by its own noon, two towns just a little way apart would have slightly different times. Long ago, that is exactly what happened. Royal Museums Greenwich explains that in Britain, Bristol kept time 10 minutes behind Greenwich and Cardiff 13 minutes behind.

Because Earth rotates 360 degrees in 24 hours, 15 degrees of longitude correspond to one hour of time. NOAA uses this to explain how navigators found longitude: comparing local noon with a clock set to Greenwich, each hour of difference meant 15 degrees east or west.

Until the mid-19th century, Royal Museums Greenwich notes, almost every town kept its own local time defined by the Sun, with no national or international conventions; Bristol Mean Time ran 10 minutes behind Greenwich Mean Time and Cardiff 13 minutes behind. Even local solar time is not perfectly regular: the interval between successive meridian crossings of the Sun varies over the year, which is why Greenwich Mean Time was defined as the yearly average, or mean.

Long ago, every town had its own time.

Then trains came. Trains need timetables. So places agreed to share the same time.

When railways spread, having a different time in every town became a real problem. In the United States, the Smithsonian explains, two trains on the same track could be using two different times, which made travel unsafe.

Britain's railway companies started using one standard time across their networks, mostly Greenwich Mean Time, and by December 1847 it was officially 'Railway Time'. In North America, the railroads set a standard time for all trains on November 18, 1883. Towns slowly switched to railroad time too.

Railways forced the change. Royal Museums Greenwich records that British railway companies introduced single standard times to make timetables less confusing; Greenwich Mean Time was adopted across Great Britain by the Railway Clearing House in December 1847 and became Britain's legal standard time in 1880.

In North America, the Smithsonian notes that town clocks, railroad timetables and the Sun often disagreed, and two trains on one track could keep different times, which meant collisions. On November 18, 1883, the railroads set a standard time for all trains. Cities gradually abandoned local time, and thirty-five years later the federal government began to enforce standard time across the United States. Britannica sums it up: time zones were introduced in the late 19th century as railways connected places with differing local times.

The world's clocks are counted from one place: Greenwich, in London.

A line on the map runs through it. It is called the prime meridian.

To share one system, the world needed a starting line. In 1884, at a meeting in Washington, D.C., called the International Meridian Conference, representatives chose a line through the telescope at the Royal Observatory in Greenwich, England, as 0 degrees longitude.

Royal Museums Greenwich says many sea charts already used Greenwich, and the United States had already based its own time zones on it. Greenwich Mean Time became the world's standard. Since 1972 it has been replaced by Coordinated Universal Time, or UTC, kept by atomic clocks.

NOAA describes the 1884 International Meridian Conference in Washington, D.C., which chose the meridian through the telescope at the Royal Observatory, Greenwich. Royal Museums Greenwich gives two main reasons: the United States had already chosen Greenwich as the basis of its national time zone system, and in the late 19th century 72 percent of the world's commerce depended on sea charts using Greenwich as the prime meridian.

GMT was the international standard of civil time from 1884 until 1972. Its successor, UTC, was devised in 1970 by technical experts within the International Telecommunication Union, who chose the abbreviation UTC so as not to favor English (CUT) or French (TUC), according to NIST. UTC is based on International Atomic Time, computed from more than 300 atomic clocks in more than 60 laboratories, and is adjusted by leap seconds to stay in step with Earth's rotation.

On a map, time zones are not straight stripes.

They bend and wiggle. They often follow the edges of countries and states.

If time zones were drawn perfectly, each would be a straight stripe 15 degrees wide from pole to pole. But NOAA explains that real time zone boundaries depend on local laws. Countries and states choose the time they use, so the lines bend to follow borders.

That means neighbors can share a clock even if the Sun is a little different for them. Royal Museums Greenwich notes that Greenwich Mean Time is the name of the time zone used by some countries in Africa and Western Europe, including Iceland all year round.

Britannica's 15-degree band is the ideal; on real maps the boundaries are political. NOAA states that actual date and time zone boundaries depend on local laws, and that even the International Date Line, based on the 180-degree antimeridian, zigzags around borders.

In the United States, NIST notes that time zones and daylight saving time are regulated by the U.S. Department of Transportation, not by NIST. Rules also differ inside one country: Hawaii, American Samoa, Guam, Puerto Rico, the Virgin Islands and Arizona do not observe daylight saving time, except for the Navajo Indian Reservation in Arizona, which does. Royal Museums Greenwich adds that GMT is still Britain's legal time in winter and is the zone name used by some countries in Africa and Western Europe, including Iceland all year round.

In some places, people move their clocks one hour forward in spring.

In the fall, they move them back again. This gives more light in summer evenings.

Many places also change their clocks during part of the year. This is called daylight saving time. NIST explains that in the United States, clocks jump ahead one hour in March and go back one hour in November: 'spring forward, fall back'. It moves an hour of daylight from the morning to the evening.

In 2026, daylight saving time in the United States runs from March 8 to November 1. It lasts 238 days, about 65 percent of the year.

Daylight saving time (DST) shifts a zone's offset by one hour for part of the year. NIST states that in the United States DST was formally introduced in 1918 and now begins at 2:00 a.m. on the second Sunday of March (the clock skips from 2 to 3 a.m.) and ends at 2:00 a.m. on the first Sunday of November (1 a.m. is repeated). In 2026 that means March 8 to November 1.

The current rules came from the Energy Policy Act of 2005, which took effect in 2007 and lengthened DST by about a month, to 238 days, or about 65 percent of the year, in the interest of reducing energy consumption. NIST's time services always broadcast UTC; your device's operating system applies the local time zone and DST rules.