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The International Date Line (IDL) is an imaginary line on the surface of the Earth that runs from the north to the south pole and demarcates backward calendar day from the next. It passes through the middle of the Pacific Ocean, roughly following the 180° longitude but it deviates to pass around some territories and island groups.
The IDL is on the opposite side of the Earth to the Prime Meridian. The Prime Meridian is used to define Universal Time and is the meridian from which all other time zones are calculated. Time zones to the east of the Prime Meridian are in advance of UTC (up to UTC+14); time zones to the west are behind UTC (to UTC-12). The IDL is the line between those highest (up to UTC+14) and those lowest (down to UTC−12) time zones.
The IDL and the moving point of midnight separate the two calendar days that are current somewhere on Earth. However, during a two-hour period between 10:00 and 11:59 (UTC) each day, three different calendar days are in use. This is because of daylight saving in the UTC+12 zone and the use of additional date-shifted time zones in areas east of the 180th meridian. These additional time zones result in the standard time and date in some communities being 24 or 25 hours different from the standard time and date in others.
A traveler crossing the IDL eastbound subtracts one day, or 24 hours, so that the calendar date to the west of the line is repeated after the following midnight. Crossing the IDL westbound results in 24 hours being added, advancing the calendar date by one day. The IDL is necessary to have a fixed, albeit arbitrary, boundary on the globe where the calendar date advances in the westbound direction.
For parts of its length, the IDL follows the meridian of 180° longitude, roughly down the middle of the Pacific Ocean. To avoid crossing nations internally it deviates around the far east of Russia and around various island groups in the Pacific. These various deviations (east or west) 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 bend east, to pass to the east of Russia's Wrangel Island and the Chukchi Peninsula, the easternmost part of Russian Siberia. It passes through the Bering Strait between the Diomede Islands at a distance of 1.5 km (1 mi) from each island. It then bends considerably southwest (and passes to west of 180°), passing west of St. Lawrence Island and St. Matthew Island and then passing midway between Alaska's Aleutian Islands and Russia's Commander Islands before bending southeast to return to 180°. Thus all of Siberia is to the west of the IDL and all of Alaska is to the east.
Two 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 of UTC-12 hours. The IDL circumscribes Kiribati by swinging far to the east, almost reaching the 150° 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, it returns westwards but remains east of 180°, passing between Samoa and American Samoa; accordingly, Samoa, Tokelau, Wallis and Futuna, Fiji, Tonga, Tuvalu and New Zealand's Kermadec Islands and Chatham Islands have the same date, while American Samoa, the Cook Islands, Niue, and French Polynesia are one day behind. (The IDL then bends southwest to return to 180°.)
A person who goes around the world from east to west (the same direction as Magellan's voyage) would lose one hour for every 15° of longitude crossed, and would lose 24 hours for one circuit of the globe from east to west if they did not compensate by adding 24 hours when they crossed the IDL. In contrast, a west-to-east circumnavigation of the globe gains an hour for every 15° of longitude crossed and requires subtracting 24 hours 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 days are observed at the same time in different places. For example, at UTC time Thursday 10:15, it is Wednesday 23:15 in American Samoa, (UTC-11), and Friday 00:15 in Kiritimati (UTC+14). For the first hour (UTC 10:00–10:59), this is true for both inhabited and uninhabited territories, but during the second hour (UTC 11:00–11:59) it is only true in 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, the Line Islands, and in the southern summer also Samoa. The first major city is Auckland, New Zealand.
The areas that are the first to see the daylight of a new day vary by the season. On 1 July, it is a large part of the Chukchi Peninsula, which uses UTC+12 and experiences midnight sun on this date. At New Year, the first places to see daylight are the South Pole and McMurdo Station in Antarctica, which both experience midnight sun. Both use UTC+13 as daylight saving time. At equinox, the first place to see daylight is the uninhabited Caroline Island, which is the easternmost land located west of the IDL, and among inhabited places it is Kiritimati.
All nations unilaterally determine their standard time zones, applicable only on land and adjacent territorial waters. These national zones do not extend into international waters. 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 "Zulu" time: it permits a single universal reference for time that is valid for all points on the globe at the same moment.
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 should adopt the standard time of a country if they are within its territorial waters within 12 nautical miles of land (about 22 km or 14 miles), but should 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 ships use a time zone of their own choosing.
The IDL on the map on this page and all other maps is an artificial construct of cartographers—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 and HMNAO maps, which ignores Kiribati's 1995 shift, is virtually identical to that adopted by the UK's Hydrographic Office about 1900.
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. 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 to the west side of the IDL by skipping 31 December 1844. 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.
Russia 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 3-hour ratification speech (an encyclopedic discussion of Russian America) on 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 Julian date to an American 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 west of the IDL until 1892, when 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 Monday 4 July 1892, American Independence Day.
In 2011, more than 119 years later, Samoa shifted back to west of the IDL by skipping Friday 30 December 2011. This changed the timezone from UTC−11 to UTC+13. The IDL now passes between Samoa and American Samoa, with American Samoa remaining aligned with the American date.
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.
Kwajalein atoll in the Marshall Islands District of the Trust Territory of the Pacific Islands (1947–1986) and its successor, the Marshall Islands, has been administered by the United States and used the Hawaiian date, so was east of the International Date Line. Kwajalein advanced to the west of the IDL by skipping 21 August 1993 to match the dates used by the rest of the Marshall Islands at the request of its government. As a result of 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 (with a remaining difference of two hours due to the two time zones involved, UTC+12 and UTC-10).
The Republic of Kiribati, in the Central Pacific, introduced a change of date for its eastern half on 1 January 1995, from time zones UTC−11 and UTC−10 to UTC+13 and UTC+14. Before this, the country was divided by the IDL. After the change, the IDL in effect moved eastwards to go around this country.
As a British colony, Kiribati was centred in the Gilbert Islands, just west of the old IDL. Upon independence in 1979, it acquired from the United States the Phoenix and Line Islands, east of the IDL, and the country straddled the IDL. Government offices on opposite sides of the line could only conduct routine business communications by radio or telephone on the four days of the week which were weekdays on both sides. This anomaly was eliminated by the 1995 change.
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 follows the legal calendar, and the Christian churches recognize the authority of the IDL. However, one important issue exists in some Orthodox countries where the Julian calendar is followed for religious purposes, but the Gregorian calendar for civilian purposes. The two calendars have the same weekdays, so the main issue is when to celebrate Easter, Christmas, and other main holidays.
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 Sabbath on the "new" 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.
The appropriate local date for holding the Friday prayer (Jumu'ah) can be a question in Islam in the Pacific region, since the date line used could be, for example, the IDL or 140°W (opposite Mecca). Hawaii (157°W) follows the U.S. date for Friday prayer.
The concept of an international date line is first mentioned in a 12th-century Talmudic commentary which seems to indicate that the day changes in an area where the time is six hours ahead of Jerusalem (90 degrees east of Jerusalem, a line running through the Philippines). This line, which he refers to as the K'tzai Hamizrach (the easternmost line), is used to calculate the day of Rosh Hashanah, the Jewish New Year. According to some sources it is alluded to in both the Talmud (Rosh Hashanah and Eruvin) and in the Jerusalem Talmud. The contemporary Kuzari seems to agree with this ruling.
The date line poses a problem for religious travellers relative to the day on which to observe Shabbat and Holidays. Shabbat is on the seventh day of the week, which is constant if one stays on the same side of the date line. The problem occurs when a Jewish traveller crosses the line and for whom it is Friday but for the place the traveller is visiting, it is Saturday. There are several opinions regarding where exactly the date line is according to Jewish law.
The halachic ruling of Rabbi Moshe Heinemann, Rabbinic Administrator of the Star-K, is as follows: In New Zealand and Japan, the local Saturday is according to majority opinion Shabbat, and it should therefore be fully observed as Shabbat, with Shabbat prayers, etc. However, since according to the Chazon Ish, Shabbat is on the local Sunday, one should not perform any Shabbat Torah prohibitions on Sunday. Nevertheless, on Sunday, one should pray the regular weekday prayers, donning tefillin during morning prayers.
Rabbi Yechiel Michel Tukatzinsky ruled that the International Date Line would be 180 degrees from the prime meridian, which would pass through Jerusalem instead of Greenwich. That would mean that the International Date Line, rather than being at 180, would be at 145W. Thus, only the areas between 180 and 145W would not observe Shabbat on the local Saturday. A third opinion holds there is no International Date Line and Jews should follow local custom for Saturday (Chabad). Thus combining the Chazon Ish with R' Tukatzinsky produces confusion in the entire area between the Asian coast and Alaskan coast. The Chazon Ish would have no problems with Hawaii and R' Tukatzinsky would have no problems with Samoa.
In Hawaii (157°W), Saturday is Shabbat according to the majority opinion. Therefore, the local Saturday is fully observed as Shabbat. The day known locally as Friday is Shabbat according to the minority opinion, and one should not perform Shabbat Torah prohibitions on that day. Cooking for Shabbat should therefore be done on Thursday. Where there are no other Jews or tradition of keeping Shabbat on a particular day, there is no clear majority opinion and Shabbat is kept 7 days from the person's last Saturday or as soon as they realise, if they do not know what day it is.
In the following locations, Shabbat is observed on the local Saturday, and a second day is not necessary: Australia, China, Mainland Russia, Taiwan, Hong Kong, Mainland Alaska, and Manila and other areas of the Philippines west of 125.2°E.
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's writings indulge in increasingly confused speculation of 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 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 21 December, and that he has lost the wager for 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 surface 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, whilst 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.