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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 one 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 International Date line is on the opposite side of the Earth to the Prime Meridian. The Prime Meridian helps 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).
Mostly, the International Date Line 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 prevent the earth from observing a single date for the instant when midnight crosses the IDL. It also results 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 International Date Line eastbound subtracts one day, or 24 hours, so that the calendar date to the west of the line is repeated. Crossing the IDL westbound results in 24 hours being added, advancing the calendar date by one day. The International Date Line 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 International Date Line follows the meridian of 180° longitude, roughly down the middle of the Pacific Ocean. To avoid crossing nations internally the line deviates around the far east of Russia and then 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.
From the north, the date line first deviates to the east of 180° to pass to the east of Russia's Wrangel Island and the Chukchi Peninsula which is the easternmost part of Russian Siberia. The date line then passes through the Bering Strait between the Diomede Islands at a distance of 1.5 km (1 mi) from each island. The line then bends considerably southwest, passing west of St. Lawrence Island and St. Matthew Island. It then passes midway between Alaska's Aleutian Islands and Russia's Commander Islands before returning southeast to 180°. Thus all of Siberia is to the west of the International Date Line, and all of Alaska is to the east of that line.
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 date line then circumscribes the country of 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, the date line returns westwards but remains east of 180°, passing between Samoa and American Samoa; accordingly, Samoa, Tokelau, Wallis and Futuna, Fiji, Tonga, and New Zealand's Kermadec Islands and Chatham Islands have the same date, while American Samoa, the Cook Islands, Niue, and French Polynesia are all one day earlier.
A person who flies (or sails) 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 cross the International Date Line. In contrast, a west-to-east circumnavigation of the globe requires subtracting 24 hours when crossing the international date line. The International Date Line 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 all islands that use UTC+14, that is the Line Islands and Tokelau, 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 the McMurdo Station in Antarctica, which both experience midnight sun at this time. Both use UTC+13 as daylight saving time. The first place to see daylight at equinox is the uninhabited Caroline Island which is the easternmost land west of the International Date Line.
All nations unilaterally determine their standard time zones, which are 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 straight line segments and their junctions of the International Date Line drawn by cartographers. Indeed, 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 and universal reference for time that is valid for all points on the globe at the same moment.
The nautical date line, which is not the same as the International Date Line, 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, for example, 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 twelve 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 territorial waters. In reality, ships use these time zones only for radio communication and similar purposes. For internal purposes, such as work and meal hours, passenger events, and facilities opening hours, ships use a time zone of their own choosing.
The International Date Line drawn on the map on this page and all other maps is now and always has been an artificial construct of cartographers—the precise course of the cartographer's 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 International Date Line drawn on the 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.
The Philippines, as part of the New Spain, long had its most important communication with Acapulco in Mexico, and was accordingly placed on the east side of the date line, despite being at the western edge of the Pacific Ocean. 00:01 Tuesday in London was 17:21 Monday in Acapulco and about 08:05 Monday in Manila. During the 1840s, trade interests turned to China, the Dutch East Indies and adjacent areas, and the Philippines was changed to the west side of the date line. Monday, 30 December 1844 (ending up as a 365-day year, despite being a leap year) was followed by Wednesday, 1 January 1845.
Russia settled northwest North America from Siberia, that is, 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, that is, from the east with its own Gregorian calendar (adopted in 1752 while a British colony). The transfer ceremony occurred on the day that the commissioners appointed by the governments of Russia and the United States for that purpose happened to arrive by ship (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—today divided into Samoa and American Samoa—were west of the date line until 1892, when King Malietoa Laupepa was persuaded by American traders to adopt the American date, being three hours behind California, to replace the former Asian date, being four hours ahead of Japan. The change was made by repeating Monday 4 July 1892, American Independence Day.
In 2011, more than 119 years after that change was made, Samoa shifted back to west of the date line by skipping Friday 30 December 2011. This changed the timezone from UTC−11 to UTC+13 (and from UTC−10 to UTC+14 during daylight saving time). The International Date Line 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 different days meant only four days of the week were shared workdays.
Tokelau, a territory of New Zealand north of Samoa that previously used UTC−10, also crossed the date line at the same time in 2011 in order to follow Samoa (where the only ferry connection went), and now uses UTC+14.
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 date line. After the change, the date line in effect moved eastwards to go around this country.
As a British colony, Kiribati was centered in the Gilbert Islands, just west of the old date line. Upon independence in 1979, the new republic acquired from the United States the Phoenix and Line Islands, east of the date line, and the country found itself straddling the date line. 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 date line change.
As a consequence of the 1995 date line change, Kiribati's easternmost territory, the Line Islands, including the inhabited island of Kiritimati (Christmas Island), started the year 2000 before any other country on Earth, a feature the Kiribati government capitalized upon as a potential tourist draw. But as of 2005,[old info] most world atlases ignored the Kiribati dateline shift and continued to represent the International Date as a straight line in the Kiribati area.
Generally, the Christian calendar follows the legal calendar, and the Christian date line is on the same place as the international date line. 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 seventh-day Sabbath) observe Sabbath on an official Sunday due to an anomaly in the International Date Line which places the line east of Tonga; as Tonga lies east of the meridian of 180° longitude, Sunday as observed in Tonga (as with Kiribati, Samoa, and parts of Fiji and Tuvalu) is considered for this purpose to be the same day as Saturday observed in most other places. Most Samoan Seventh Day Adventists planned to continue to observe Sabbath on the official Sunday after Samoa's crossing the date line in December 2011, but the church in Samatau village decided to adjust and observe Sabbath on the "new" Saturday. The Samoan Independent Seventh-day Adventist Church, which is an independent church from the worldwide Seventh-day Adventist Conference church, has decided to continue worshiping on Saturday, after a 6 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 International Date Line, or be 140°W (opposite to 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 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.
In Hawaii, 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. Determining the majority opinion on the Aleutian Islands or South Pacific Islands, including Fiji and American Samoa, is complicated.[why?].
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 International Date Line. Unable to swim, the protagonist's writings indulge in increasingly confused speculation of the physical, metaphysical and religious import of the date line.
The date line 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. As a consequence of a wager made with some other club members, Fogg had to return by 8:45 pm on Saturday, 21 December 1872. Being suspected of a bank robbery, he is trapped for a whole day and reaches London believing it was later than Saturday evening. Believing he had lost the bet and his wealth, early the next morning he sends his servant to arrange a wedding with Aouda, whom he saved from being burned alive in India. The servant returns yelling that the wedding was refused because it was Saturday, not Sunday. Only then does Phileas Fogg realize that he had not accounted for the date line and had actually traveled for 79 days, not 80. Eventually he arrives at the Reform Club just in time to win the bet, his fortune back, and Aouda's love.