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What is the International Date Line?

The International Date Line is an imaginary boundary that runs from the North to the South Pole, marking the divide where one day ends and the next begins. It ensures global timekeeping consistency. As you cross this line, you either step into yesterday or tomorrow. Intrigued? Discover how this invisible line shapes our understanding of time across the world.
Mary McMahon
Mary McMahon
Mary McMahon
Mary McMahon

The International Date Line (IDL), is an imaginary line that runs roughly along the 180° line of longitude, in the middle of the Pacific Ocean. International convention accepts the line as the location where one day is divided from the next, with the area in the Eastern Hemisphere one day ahead of the Western Hemisphere. This line is necessary to address certain oddities that occur during travel; people going all the way around the world perceive themselves either gaining or losing a day, depending on which direction they traveled in.

How Time Zones Work

A global map including time zone divisions.
A global map including time zone divisions.

In theory, the world could be divided up into 24 time zones, one for each hour of the day. Time zones are actually set by each country independently, however, and local time does not necessarily match these ideal standard time divisions. Ideally, the time zones are designed so that local noon falls when the sun is roughly overhead. Traveling west, a person would lose an hour for every 15° of longitude; while traveling east, towards the sun, he or she gains an hour.

The International Date Line winds around some inhabited Alaskan islands so they are not on a different day than the rest of the state.
The International Date Line winds around some inhabited Alaskan islands so they are not on a different day than the rest of the state.

Time zones are described in terms of their relationship to the prime meridian, which falls at 0° of longitude in Greenwich, England. If someone was 45° away from Greenwich, he or she would experience a three-hour time difference; 45° to the west would mean that the person was in Coordinated Universal Time (UTC) -3, while someone 45° to the east would be in UTC +3. The International Date Line falls directly opposite the Prime Meridian in the time zones UTC +12 and UTC -12. Coordinated Universal Time was formerly known as Greenwich Mean Time (GMT), and is still sometimes written with this abbreviation.

How the IDL Location Was Chosen

Confusion of days and schedules may happen when traveling across the International Date Line.
Confusion of days and schedules may happen when traveling across the International Date Line.

The location of the International Date Line was chosen because it is in a relatively uninhabited area of the Pacific. The presence of inhabited areas — including Alaska, Russia, and a number of islands — mean that the line deviates in some areas. If the line was straight, the eastern most part of Russia would be a day behind the rest of the country, while a number of islands that are part of Alaska would be a day ahead of the state. This would likely result in confusion and administrative chaos.

Moving the Line

Sculpture located at the Prime Meridian in Greenwich.
Sculpture located at the Prime Meridian in Greenwich.

The line has been moved several times put a country on one or the other side. The islands of the Philippines, for example, were on the eastern side of the Date Line — with the Western Hemisphere — until the mid-19th century, despite sitting at about 120° E longitude. At the end of 2011, the islands of Samoa and Tokelau, which are very close to the line, shifted from the east to the west side of the line, rejoining the Eastern Hemisphere. Such changes are often made for economic reasons; Samoa and Tokelau shifted because of growing trade connections with Australia and New Zealand.

Crossing the Line

Most airlines and airports take the International Date Line into account when issuing flight times.
Most airlines and airports take the International Date Line into account when issuing flight times.

Traveling across the International Date Line can be confusing, especially for people taking a short trip, such as from Fiji to Hawaii. According to the clock, a traveler would end up arriving in Hawaii before he or she had left Fiji, because Fiji is a day ahead of Hawaii, and two hours behind — when it is noon in Hawaii, it is 10:00 AM in Fiji on the next day. Confusions of days and schedules do sometimes lead to mishaps, but most airlines and travel agencies keep the line in mind when informing travelers about schedules, expected arrival times, and itineraries.

Frequently Asked Questions

What is the International Date Line and where is it located?

The International Date Line (IDL) is an imaginary line on the Earth's surface that roughly follows the 180° longitude, opposite the Prime Meridian. It serves as the "line of demarcation" between calendar dates. When crossed from west to east, travelers subtract a day, and when crossed from east to west, they add a day. The IDL zigzags around some territories and island groups to align with political and territorial boundaries.

Why does the International Date Line deviate from the 180° longitude?

The International Date Line deviates from the 180° longitude to accommodate the political and cultural needs of the countries it would otherwise split. This ensures that entire countries and closely associated islands follow the same calendar date. For instance, the IDL bends eastward around Russia and westward around the Aleutian Islands to keep them on the same day as the rest of their respective countries.

How does the International Date Line affect time zones?

The International Date Line affects time zones by serving as the reference point for where the date changes. Time zones to the west of the IDL are always one day ahead of those to the east. However, the IDL does not affect the actual time (hours and minutes), which is determined by separate time zone divisions. Each time zone is typically an hour apart, and they adjust according to the IDL's date change.

Can the position of the International Date Line change?

Yes, the position of the International Date Line can change. It is not fixed by international law but rather by agreements between nations. Changes have occurred to accommodate the needs of certain countries or territories. For example, Samoa shifted the IDL in 2011 to align its weekdays with major trading partners like Australia and New Zealand, effectively "skipping" a day in the process.

What are some interesting facts about the International Date Line?

One interesting fact about the International Date Line is that despite its significance in timekeeping, it has no international legal status. Additionally, the IDL is the reason why Kiribati's Line Islands are the first to welcome the New Year, despite being relatively close to Hawaii, which celebrates much later. Moreover, travelers crossing the IDL can experience the unique situation of "time travel," where they gain or lose an entire day.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a CulturalWorld researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Learn more...
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a CulturalWorld researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Learn more...

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Discussion Comments

anon997692

I am not sure, but I was told that Russia has several time zones, but they all go by Moscow time.

mentirosa

anon265487 - One could live in fantasy land or in real world. In fantasy land anything is possible. Fortunately, or unfortunately in the real world there are certain facts that we can not get around. It just is.

I am curious, how exactly would you make those changes? For example if in Vienna, Austria it is 12 noon, sun is shining, and in Honolulu, Hawaii it is midnight, dark, most people are asleep, how would you handle it? What date and time would it be in each place according to you?

anon265487

We need to get rid of time zones and the date line. The earth is more connected than ever. The current system is a vestige from the days of the railroad. One world, one time, one date.

anon154279

So, if I lived in hawaii, and fiji, and moved back and forth systematically, I would live longer than someone staying in one place.

anon133596

Hey does anyone know, what time is it at the Prime Meridian when the sun is directly over the international date line?

anon133456

The dateline was not chosen because it is a convenient location. It is 180 degrees from greenwich -- the opposite side of the earth from the Prime Meridian.

anon50993

thanks!

anon4512

now, where exactly is it located?

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    • A global map including time zone divisions.
      By: Ruslan Olinchuk
      A global map including time zone divisions.
    • The International Date Line winds around some inhabited Alaskan islands so they are not on a different day than the rest of the state.
      By: juan35mm
      The International Date Line winds around some inhabited Alaskan islands so they are not on a different day than the rest of the state.
    • Confusion of days and schedules may happen when traveling across the International Date Line.
      By: olly
      Confusion of days and schedules may happen when traveling across the International Date Line.
    • Sculpture located at the Prime Meridian in Greenwich.
      By: Štěpán Roh
      Sculpture located at the Prime Meridian in Greenwich.
    • Most airlines and airports take the International Date Line into account when issuing flight times.
      By: Pavel Losevsky
      Most airlines and airports take the International Date Line into account when issuing flight times.