The IDL is roughly based on the meridian of 180° longitude, roughly down the middle of the Pacific Ocean, and halfway around the world from the Greenwich meridian. In many places, the IDL follows the 180° meridian exactly. In other places, however, the IDL deviates east or west away from that meridian. These various deviations generally accommodate the political and/or economic affiliations of the affected areas.
Proceeding from north to south, the first deviation of the IDL from 180° is to pass to the east of Wrangel Island and the Chukchi Peninsula, the easternmost part of Russian Siberia. (Wrangel Island lies directly on the meridian at 71°32′N 180°0′E, also noted as 71°32′N 180°0′W.) It then passes through the Bering Strait between the Diomede Islands at a distance of 1.5 kilometres (0.93 mi) from each island at 168°58′37″ W. It then bends considerably west of 180°, passing west of St. Lawrence Island and St. Matthew Island.
The IDL crosses between the U.S. Aleutian Islands (Attu Island being the westernmost) and the Commander Islands, which belong to Russia. It then bends southeast again to return to 180°. Thus, all of Russia is to the west of the IDL, and all of the United States is to the east except for the insular areas of Guam, the Northern Mariana Islands, and Wake Island.
The IDL remains on the 180° meridian until passing the equator. Two US-owned uninhabited atolls, Howland Island and Baker Island, just north of the equator in the central Pacific Ocean (and ships at sea between 172.5°W and 180°), have the latest time on Earth (UTC−12 hours).
The IDL circumscribes Kiribati by swinging far to the east, almost reaching the 150°W meridian. Kiribati's easternmost islands, the southern Line Islands south of Hawaii, have the most advanced time on Earth, UTC+14 hours. South of Kiribati, the IDL returns westwards but remains east of 180°, passing between Samoa and American Samoa.
In much of this area, the IDL follows the 165°W meridian. Accordingly, Samoa, Tokelau, Wallis and Futuna, Fiji, Tonga, Tuvalu and New Zealand's Kermadec Islands and Chatham Islands are all west of the IDL and have the same date. American Samoa, the Cook Islands, Niue, and French Polynesia are east of the IDL and one day behind.
The IDL then bends southwest to return to 180°. It follows that meridian until reaching Antarctica, which has multiple time zones. Conventionally, the IDL is not drawn into Antarctica on most maps. (See § Cartographic practice and convention below.)
A person who goes around the world from east to west (the same direction as Magellan's voyage) would gain or set their clock back one hour for every 15° of longitude crossed, and would gain 24 hours for one circuit of the globe from east to west if they did not compensate by setting their clock forward one day when they crossed the IDL. In contrast, a west-to-east circumnavigation of the globe loses an hour for every 15° of longitude crossed but gains back a day when crossing the IDL. The IDL must therefore be observed in conjunction with the Earth's time zones: on crossing it in either direction, the calendar date is adjusted by one day.
For the two hours between 10:00 and 11:59 UTC each day, three different calendar dates are observed at the same time in different places on Earth. For example, at 10:15 UTC Thursday, it is 23:15 Wednesday in American Samoa (UTC−11), Thursday in most of the world, and 00:15 Friday in Kiritimati (UTC+14).
During the first hour (UTC 10:00–10:59), all three calendar dates include inhabited places. During the second hour (UTC 11:00–11:59) one of the calendar dates is limited to an uninhabited maritime time zone twelve hours behind UTC (UTC−12).
According to the clock, the first areas to experience a new day and a New Year are islands that use UTC+14. These include portions of the Republic of Kiribati, including Millennium Island in the Line Islands, as well as Samoa during the southern summer. The first major cities to experience a new day are Auckland and Wellington, New Zealand (UTC+12; UTC+13 with daylight saving time).
A 1995 realignment of the IDL made Caroline Island one of the first points of land on Earth to reach January 1, 2000 on the calendar (UTC+14). As a result, this atoll was renamed Millennium Island.
The areas that are the first to see the daylight of a new day vary by the season. Around the June solstice, the first area would be anyplace within the Kamchatka Time Zone (UTC+12) that is far enough north to experience midnight sun on the given date. At the equinoxes, the first place to see daylight would be the uninhabited Millennium Island in Kiribati, which is the easternmost land located west of the IDL.
Near the December solstice, the first places would be Antarctic research stations using New Zealand Time (UTC+13) during summer that experience midnight sun. These include Amundsen-Scott South Pole Station, McMurdo Station, Scott Base and Mario Zucchelli Station.
There are two ways time zones and thereby the location of the International Date Line are determined, one on land and adjacent territorial waters, and the other on open seas.
All nations unilaterally determine their standard time zones, applicable only on land and adjacent territorial waters. This date line can be called de facto since it is not based on international law, but on national laws. These national zones do not extend into international waters.
The nautical date line, not the same as the IDL, is a de jure construction determined by international agreement. It is the result of the 1917 Anglo-French Conference on Time-keeping at Sea, which recommended that all ships, both military and civilian, adopt hourly standard time zones on the high seas. The United States adopted its recommendation for U.S. military and merchant marine ships in 1920. This date line is implied but not explicitly drawn on time zone maps. It follows the 180° meridian except where it is interrupted by territorial waters adjacent to land, forming gaps—it is a pole-to-pole dashed line. The 15° gore that is offset from UTC by 12 hours is bisected by the nautical date line into two 7.5° gores that differ from UTC by ±12 hours.
Ships are supposed to adopt the standard time of a country if they are within its territorial waters within 12 nautical miles (14 mi; 22 km) of land, then revert to international time zones (15° wide pole-to-pole gores) as soon as they leave. In reality, ships use these time zones only for radio communication and similar purposes. For internal purposes, such as work and meal hours, ships use a time zone of their own choosing.
The IDL on the map on this page and all other maps is based on the de facto line and is an artificial construct of cartographers, as the precise course of the line in international waters is arbitrary. The IDL does not extend into Antarctica on the world time zone maps by the United States Central Intelligence Agency (CIA) or the United Kingdom's Her Majesty's Nautical Almanac Office (HMNAO). The IDL on modern CIA maps now reflects the most recent shifts in the IDL (see § Historical alterations below). The current HMNAO map does not draw the IDL in conformity with recent shifts in the IDL; it draws a line virtually identical to that adopted by the UK's Hydrographic Office about 1900. Instead, HMNAO labels island groups with their time zones, which do reflect the most recent IDL shifts. This approach is consistent with the principle of national and nautical time zones: the islands of eastern Kiribati are actually "islands" of Asian date (west side of IDL) in a sea of American date (east side of IDL).
No international organization, nor any treaty between nations, has fixed the IDL drawn by cartographers: the 1884 International Meridian Conference explicitly refused to propose or agree to any time zones, stating that they were outside its purview. The conference resolved that the Universal Day, midnight-to-midnight Greenwich Mean Time (now known as Coordinated Universal Time, or UTC), which it did agree to, "shall not interfere with the use of local or standard time where desirable". From this comes the utility and importance of UTC or "Z (Zulu)" time: it permits a single universal reference for time that is valid for all points on the globe at the same moment.
As part of New Spain, the Philippines long had its most important communication with Acapulco in Mexico and was accordingly on the east side of the IDL despite being at the far western edge of the Pacific Ocean. From 1521 to 1844, the Philippines was one day behind its Asian neighbors (1521 was the first European visit, when Ferdinand Magellan claimed the area for Spain. Colonization began in 1565). After Mexico gained its independence from Spain in 1821, Philippine trade interests turned to Imperial China, the Dutch East Indies and adjacent areas, so the Philippines decided to shift back to the west side of the IDL by removing Tuesday, 31 December 1844 from its calendar. The change was also applied to Mariana Islands, Guam and Caroline Islands since they also belonged to Spain. Because of this change, New Year's Eve happened on Monday, 30 December 1844. Effective Wednesday, 1 January 1845, the Philippines, Guam, Mariana Islands and Caroline Islands switched back to Asian date. Western publications were generally unaware of this change until the early 1890s, so erroneously gave the International Date Line a large western bulge for the next half century.
The Russian Empire settled northwest North America from Siberia, from the west with its own Julian calendar (it did not adopt the Gregorian calendar until 1918). The United States purchased Russian America while based in the contiguous United States, from the east with its own Gregorian calendar (adopted in 1752 while several British colonies). The transfer ceremony occurred on the day that the commissioners appointed by the governments of Russia and the United States for that purpose arrived via the USS Ossipee at New Archangel (Sitka), the capital of Russian America. The United States recorded this date as Friday, 18 October 1867 (Gregorian), now known as Alaska Day, whereas the Russian governor, who had remained in New Archangel, would have recorded it as Saturday, 7 October 1867 (Julian). Senator Charles Sumner stated during his three-hour ratification speech (an encyclopedic discussion of Russian America) on Tuesday, 9 April 1867, that this day of the week and calendar discord should be changed. Because the transfer of ownership officially occurred at 3:30 p.m. Sitka mean solar time (time zones were not yet in use), that was the date and time that Alaska changed from an Asian and Julian date to an American and Gregorian date. If the transfer had occurred at the preceding midnight then Friday, 6 October 1867 (Julian) would have been followed by Friday, 18 October 1867 (Gregorian), a duplicate day with a 12-day difference appropriate both for changing from an Asian date to an American date (equivalent to moving the IDL from the east to the west of Alaska) and for changing from the Julian calendar to the Gregorian calendar during the 19th century.
The Samoan Islands, now divided into Samoa and American Samoa, were on the west side of the IDL until 1892. In that year, King Malietoa Laupepa was persuaded by American traders to adopt the American date (three hours behind California) to replace the former Asian date (four hours ahead of Japan). The change was made by repeating 4 July 1892, American Independence Day.
In 2011, Samoa shifted back to the west side of the IDL by removing Friday, 30 December 2011 from its calendar. This changed the timezone from UTC−11 to UTC+13. Samoa made the change because Australia and New Zealand have become its biggest trading partners, and also have large communities of expatriates. Being 21 hours behind made business difficult because having weekends on backward days meant only four days of the week were shared workdays.
The IDL now passes between Samoa and American Samoa, which remains on the east (American) side of the line.
Tokelau is a territory of New Zealand north of Samoa whose principal transportation and communications links with the rest of the world pass through Samoa. For that reason, Tokelau crossed the IDL along with Samoa in 2011.
Kwajalein atoll, like the rest of the Marshall Islands, passed from Spanish to German to Japanese control during the nineteenth and twentieth centuries. During that period it was west of the IDL. Although Kwajalein formally became part of the Trust Territory of the Pacific Islands with the rest of the Marshalls after World War II, the United States established a military installation there. Because of that, Kwajalein used the Hawaiian date, so was effectively east of the International Date Line (unlike the rest of the Marshalls). Kwajalein returned to the west side of the IDL by removing Saturday, 21 August 1993 from its calendar to match the dates used by the rest of the Marshall Islands, at the request of its government. Along with the shift, Kwajalein's work week was changed to Tuesday through Saturday to match the Hawaiian work week of Monday through Friday on the other side of the IDL.
As a British colony, the now-Republic of Kiribati was centred in the Gilbert Islands, just west of the IDL of the time. Upon independence in 1979, it acquired the Phoenix and Line Islands, east of the IDL, from the United States. As a result, the country straddled the IDL. Government and commercial concerns on opposite sides of the line could only conduct routine business by radio or telephone on the four days of the week which were weekdays on both sides. To eliminate this anomaly, Kiribati introduced a change of date for its eastern half on Sunday, 1 January 1995. After the change, the IDL in effect moved eastwards to go around the entire country. Strictly legal, the 1917 nautical IDL convention is still valid. When the land time zone says it's Monday, these islands would form enclaves of Monday in an ocean which has Sunday. Maps are usually not drawn this way.
As a consequence of the 1995 change, Kiribati's easternmost territory, the Line Islands, including the inhabited island of Kiritimati (Christmas Island), started the year 2000 before any other country, a feature the Kiribati government capitalized upon as a potential tourist draw.
Generally, the Christian calendar and Christian churches recognize the authority of the IDL. Christmas for example, is celebrated on 25 December (according to either the Gregorian or the Julian calendar, depending upon which of the two is used by the particular church) as that date falls in countries located on either side of the Date Line. Thus, whether it is Western Christmas or Orthodox Christmas, Christians in Samoa, immediately west of the Date Line, will celebrate the holiday a day before Christians in American Samoa, which is immediately east of the Date Line.
A problem with the general rule above arises in certain Christian churches that solemnly observe a Sabbath day as a particular day of the week, when those churches are located in countries near the Date Line. Notwithstanding the difference in dates, the same sunrise happened over American Samoa as happens over Samoa a few minutes later, and the same sunset happens over Samoa as happened over American Samoa a few minutes earlier. In other words, the secular days are legally different but they are physically the same; and that causes questions to arise under religious law.
Because the Date Line was an arbitrary imposition, the question can arise as to which Saturday on either side of the Date Line (or, more fundamentally, on either side of 180 degrees longitude) is the "real" Saturday. This issue (which also arises in Judaism) is a particular problem for Seventh Day Adventists, Seventh Day Baptists, and similar churches located in countries near the Date Line.
In Tonga, Seventh Day Adventists (who usually observe Saturday, the seventh-day Sabbath) observe Sunday due to their understanding of the International Date Line, as Tonga lies east of the 180° meridian. Sunday as observed in Tonga (as with Kiribati, Samoa, and parts of Fiji and Tuvalu) is considered by the Seventh-day Adventist Church to be the same day as Saturday observed in most other places.
Most Seventh Day Adventists in Samoa planned to observe Sabbath on Sunday after Samoa's crossing the date line in December 2011, but SDA groups in Samatau village and other places (approx. 300 members) decided to accept the IDL adjustment and observe the Sabbath on Saturday. Debate continues within the Seventh-day Adventist community in the Pacific as to which day is really the seventh-day Sabbath.
The Samoan Independent Seventh-day Adventist Church, which is not affiliated to the worldwide Seventh-day Adventist Church, has decided to continue worshiping on Saturday, after a six-day week at the end of 2011.
Similarly, the Islamic calendar and Muslim communities recognize the convention of the IDL. In particular, the day for holding the Jumu'ah prayer appears to be local Friday everywhere in the world.
The IDL is not a factor in the start and end of Islamic lunar months. These depend solely on sighting the new crescent moon. As an example, the fasts of the month of Ramadan begin the morning after the crescent is sighted. That this day may vary in different parts of the world is well known in Islam. (See Ramadan § Beginning.)
The concept of an international date line in Jewish law is first mentioned by 12th-century decisors. But it was not until the introduction of improved transportation and communications systems in the 20th century that the question of an international date line truly became a question of practical Jewish law.
As a practical matter, the conventional International Date Line—or another line in the Pacific Ocean close to it—serves as a de facto date line for purposes of Jewish law, at least in existing Jewish communities. For example, residents of the Jewish communities of Japan, New Zealand, Hawaii, and French Polynesia all observe Shabbat on local Saturday. However, there is not unanimity as to how Jewish law reaches that conclusion. For this reason, some authorities rule that certain aspects of Sabbath observance are required on Sunday (in Japan and New Zealand) or Friday (in Hawaii and French Polynesia) in addition to Saturday. Additionally, there are differences of opinion as to which day or days individual Jews traveling in the Pacific region away from established Jewish communities should observe Shabbat.
For individuals crossing the date line, the change of calendar date influences some aspects of practice under Jewish law. Yet other aspects depend on an individual's experience of sunsets and sunrises to count days, notwithstanding the calendar date.
The date line is a central factor in Umberto Eco's book The Island of the Day Before (1994), in which the protagonist finds himself on a becalmed ship, with an island close at hand on the other side of the IDL. Unable to swim, the protagonist indulges in increasingly confused speculation regarding the physical, metaphysical and religious import of the date line.
The concept behind the IDL (though not the IDL itself, which did not yet exist) appears as a plot device in Jules Verne's book Around the World in Eighty Days (1873). The main protagonist, Phileas Fogg, travels eastward around the world. He had bet with his friends that he could do it in 80 days. To win the wager, Fogg must return by 8:45 pm on Saturday, 21 December 1872. However, the journey suffers a series of delays and when Fogg reaches London, it's 8:50 pm on Friday, 20 December, although he believes it's Saturday, 21 December and that he has lost the wager by a margin of only five minutes. The next day, however, it is revealed that the day is Saturday, not Sunday, and Fogg arrives at his club just in time to win the bet. Verne explains:
In journeying eastward he [Fogg] had gone towards the sun, and the days therefore diminished for him as many times four minutes as he crossed degrees in this direction. There are three hundred and sixty degrees on the circumference of the earth; and these three hundred and sixty degrees, multiplied by four minutes, gives precisely twenty-four hours—that is, the day unconsciously gained. In other words, while Phileas Fogg, going eastward, saw the sun pass the meridian eighty times, his friends in London only saw it pass the meridian seventy-nine times.
Fogg had thought it was one day more than it actually was, because he had forgotten this simple fact. During his journey, he had added a full day to his clock, at the rhythm of an hour per fifteen degrees, or four minutes per degree, as Verne writes. At the time, the concept of a de jure International Date Line did not exist. If it did, he would have been made aware that it would be a day less than it used to be once he reached this line. Thus, the day he would add to his clock throughout his journey would be thoroughly removed upon crossing this imaginary line. But a de facto date line did exist since the UK, India and the US had the same calendar with different local times, and he should have noticed when he arrived to the US that the local date was not the same as in his diary (his servant Jean Passepartout kept his clock in London time, despite the tips of his surroundings).
Ceremonies aboard ships to mark a sailor's or passenger's first crossing of the Equator, as well as crossing the International Date Line, have been long-held traditions in navies and in other maritime services around the world.
The need for a temporal discontinuity on the globe can be described mathematically as following from the Borsuk-Ulam Theorem in dimension 1: It is a topological fact that there does not exist any continuous, one-to-one function mapping from a circle onto an interval.