What time zone starts the day first

Everyone on the planet wants the sun to be at its highest point in the sky (crossing the meridian) at noon. If there were just one time zone, this would be impossible because the Earth rotates 15 degrees every hour. The idea behind multiple time zones is to divide the world into 24 15-degree slices and set the clocks accordingly in each zone. All of the people in a given zone set their clocks the same way, and each zone is one hour different from the next.

In the continental United States there are four time zones (click here for a map): Eastern, Central, Mountain and Pacific. When it is noon in the Eastern time zone, it is 11 a.m. in the Central time zone, 10 a.m. in the Mountain time zone and 9 a.m. in the Pacific time zone.

All time zones are measured from a starting point centered at England's Greenwich Observatory. This point is known as the Greenwich Meridian or the Prime Meridian. Time at the Greenwich Meridian is known as Greenwich Mean Time (GMT) or Universal Time. The Eastern time zone in the United States is designated as GMT minus five hours. When it is noon in the Eastern time zone, it is 5 p.m. at the Greenwich Observatory. The International Date Line (IDL) is located on the opposite side of the planet from the Greenwich Observatory.

Why is the Greenwich Observatory such a big deal? A bunch of astronomers declared the Greenwich Observatory to be the prime meridian at an 1884 conference. What's funny is that the observatory moved to Sussex in the 1950s, but the original site remains the prime meridian.

The International Date Line (IDL) is an imaginary line on Earth's surface defining the boundary between one day and the next.

The International Date Line zigzags through the Pacific Ocean at around 180° longitude.

©timeanddate.com

The International Date Line (IDL) is located at about 180° east (or west). It is halfway around the world from the prime meridian (0° longitude), the reference point of time zones, which runs through Greenwich, UK.

The date line runs from the North Pole to the South Pole and marks the Western and Eastern Hemisphere divide. It is not straight but curves around landmasses and national borders. For example, it leans towards the east at the Bering Strait between Asia and North America, leaving Cape Dezhnev in Russia a day ahead of Cape Prince of Wales in Alaska even though they are only 80 km (50 mi) apart.

The Northern and Southern Hemisphere

Different Days on Either Side

The International Date Line is the boundary where each calendar day starts and is also known as the “Line of Demarcation” because it separates two calendar dates: When you cross the date line traveling east, you subtract a day, and if you cross the line traveling west, you add a day.

For example, Kiribati and Hawaii are on different sides of the date line. If you traveled from Kiribati's capital Tarawa to Hawaii's state capital Honolulu, you would have to turn the clock back 22 hours, effectively traveling back in time, at least on the calendar.

See the date line on our Time Zone Map

Not Always 24 Hours

Depending on which time zone the country follows, the time difference on either side of the line is not always 24 hours. For example, if you travel the 1061 km (659 mi) across the date line from Baker Island to Tokelau, you must add 25 hours, or 1 day and 1 hour.

Three Dates at the Same Time

Every day between 10:00 and 11:59 UTC, three different calendar dates are in use simultaneously on Earth.

For example, May 2 at 10:30 UTC, is 23:30 (11:30 pm) on May 1 in American Samoa (UTC−11), 06:30 (6:30 am) on May 2 in New York (UTC-4), and 00:30 (12:30 am) on May 3 in Kiritimati (UTC+14).

See it for yourself in our Time Zone Converter.

Changes in the Date Line

The International Date Line is not defined by international law, and it is up to the different countries to choose the date and time zone they want to observe.

In 1994, the Republic of Kiribati aligned the country's time zones by skipping January 1, 1995 in the portion of the country that was east of the date line. By adding 24 hours to the the clocks in the Phoenix and Line Islands, they put a dent in the date line, moving it all the way to the 150° east longitude.

In 2011, Samoa changed its time zone from UTC-11 to UTC+13 by shifting the date line to the west and removing December 30, 2011 from the calendar. They did this to facilitate trade with Australia and New Zealand, and Tokelau followed Samoa for the same reasons.

The Nautical Date Line

The International Date Line is de facto, which means that it is not based on international law and should not be confused with the Nautical Date Line, which follows the 180° meridian from the North to the South Pole. Unlike the International Date Line, whose course is determined by the countries bordering it, the Nautical Date Line is defined by international agreements.

Drawn up in 1884

The 180° meridian was selected as the basis for the International Date Line because it mostly runs through the sparsely populated Central Pacific Ocean. This was decided at the International Meridian Conference in 1884 in Washington, D.C.

Topics: Timekeeping, Time Zone, Geography

Updated March 13, 2018

By John Peterson

Each day on Earth begins at midnight in Greenwich, England, where the prime meridian is located. Originally, the prime meridian's purpose was to help ships at sea find their longitude and determine accurately their position on the globe. The calibration of chronometers – time measurement instruments – with solar time was necessary to find longitude. Determining longitude soon led to the establishment of time zones and a coordinated, international standard time. In modern times, atomic clocks have replaced solar time.

The Royal Observatory in Greenwich, England, is the key location for timekeeping worldwide. It is also located at the internationally recognized prime meridian, which is 0 degrees longitude, where each day begins at midnight. All locations on Earth are marked off east and west of the prime meridian in the same way that locations are measured north and south from the equator. The Royal Observatory was established in 1675 by King Charles II to help ships at sea calibrate their chronometers to determine longitude and location. The standard set for timekeeping, a key component in determining longitude, at Greenwich made it the timekeeper of the world.

Because solar time, as measured by a sun dial, can vary as much as 16 minutes throughout the year, a mean time must be calculated so that the marking of time can be standardized. This is known as Greenwich Mean Time, or GMT. The rotation of the Earth causes variation in solar time east to west, and noon at one location may be 3 o'clock at another. A standardized location, or prime meridian, was required to accurately calculate mean solar time and mark time differences by longitude. This process also established the 24 time zones across the globe, and the prime meridian is used as the starting point for each new day at midnight.

Historically, one of the great difficulties in ocean navigation was determining longitude. To determine longitude, a ship captain had to know the exact moment of high noon at his position at sea, in addition to high noon at a common location, or the prime meridian. This required highly calibrated chronometers to keep time, and the Royal Observatory eventually became the keeper of time, as its astronomers could accurately record high noon. But various countries chose to position their prime meridians in different locations to suit local needs, making coordination between nations difficult. To solve this problem, Greenwich, in 1884, became the official site of the prime meridian and the location for each new day and year to begin.

Keeping accurate time has become sophisticated and necessary for the complexity of the modern world. Coordinated Universal Time, or UTC, is used as the correct time worldwide, and has replaced GMT as the standard. The prime meridian is where UTC is established. While historically, astronomers set GMT using solar time, UTC is more precise and depends upon atomic clocks. Solar time can have some margin of error due to irregularities in the Earth's rotation, but atomic clocks are calibrated to be accurate to the billionth of a second.