The International Date Line (IDL) is an imaginary line on the surface of the Earth, running between the north and south poles, that demarcates one calendar day from the next. It passes through the middle of the Pacific Ocean, roughly along the 180° longitude, opposite the Prime Meridian, but it is drawn with diversions to pass around some territories and island groups.
In conjunction with the moving imaginary line opposite the sun corresponding to midnight, the IDL separates the two calendar days being used somewhere on earth most of the time, 22 hours out of each 24. Three different calendar days are in simultaneous use during a two-hour period between 10:00 and 11:59 (UTC) each day, due to the use of additional date-shifted time zones in a few areas affected by substantial deviations of the date line from the 180th meridian. The use of these additional time zones also precludes the possibility of the entire planet observing only a single date at the instant when midnight crosses the IDL.
A traveler crossing the IDL eastbound subtracts one day, or 24 hours, so that the 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 line is necessary in order to have a fixed, albeit arbitrary, boundary on the globe where the calendar date advances in the westbound direction.
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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 to pass around the far east of Russia and various island groups in the Pacific, with the location of the various deviations (east or west) generally accommodating political and/or economic affiliations of the affected areas.
In the north, the date line makes several bends in the Bering Sea region in order to stay between the eastern part of Russian Siberia and the U.S.state of Alaska, keeping the date the same within each of these areas, including their offshore islands. From the north, the date line first swings to the east of Russia's Wrangel Island and Chukchi Peninsula, 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, then passing midway between Alaska's Aleutian Islands and Russia's Commander Islands before returning southeast to 180°.
Two uninhabited atolls, Howland Island and Baker Island, just north of the equator in the central Pacific Ocean, have the latest time on Earth of UTC-12 hours, along with ships at sea between 172.5°W and 180°. 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;[1] 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.
If one flies (or sails) around the world from east to west (the same direction as Magellan's voyage), one hour is lost for every 15° of longitude crossed, losing 24 hours for one circuit of the globe; one compensates by adding 24 hours when crossing the International Date Line from east to west. In contrast, a west-to-east circumnavigation of the globe requires subtracting 24 hours when crossing the IDL. 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 two hours every day, between 10:00 and 11:59 (UTC), there are actually three different days being observed at the same time in different places. At UTC time Thursday 10:15, for example, it is Wednesday 23:15 in American Samoa, which is eleven hours behind UTC, and it is Friday 00:15 in Kiritimati (separated from American Samoa by the IDL), which is 14 hours ahead of UTC. For the first hour (UTC 10:00–10:59), this is true for inhabited territories, whereas during the second hour (UTC 11:00–11:59) it is true only in an uninhabited maritime time zone twelve hours behind UTC.
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 22km 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 IDL 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[2] or Her Majesty's Nautical Almanac Office in the United Kingdom.[3]
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.[4]
Until 1867, Alaska began Russia's day, with the date line following the partially defined border between Russian Alaska and British North America, including the colony of British Columbia, now a Canadian province. The day before the purchase of Alaska by the United States took effect, it was Friday, 6 October 1867, in the Julian calendar that was used by Russia at the time, which would have been 18 October in the Gregorian calendar used by the United States. The time in New Archangel (now Sitka, Alaska) would have been 12:00 when it was 12:02, Thursday, 17 October, at the future site of Whitehorse in Canada's Yukon Territory, and 12:49, 17 October, at the future site of Vancouver, British Columbia. With the transfer of governance, the date line was shifted westward (moving Alaska back a day), and the calendar was changed (moving Alaska ahead 12 days), and being effective at midnight the calendar moved ahead one day as well, for a net change of 12 days. Accordingly, Friday, 6 October 1867, was followed in Alaska by Friday, 18 October 1867, not by Saturday, 7 October).
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.[5][6]
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.[7] This changed the timezone from UTC−11 to UTC+13 (and from UTC−10 to UTC+14 during daylight saving time).[5] 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.[8]
Tokelau, a territory of New Zealand north of Samoa that previously used UTC−10, also crossed the date line at the same time for the same reasons and now uses UTC+14.[9]
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. As a consequence, 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, most world atlases ignored the Kiribati dateline shift and continued to represent the International Date as a straight line in the Kiribati area.[10]
In Christianity it is often considered important to hold services on the correct day of the week. 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, the Seventh Day Adventists (who usually observe the sabbath on a Saturday) observe the sabbath on a Sunday, officially due to an anomaly in the International Date Line which places the line east of Tonga, but 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.[11]
Most Samoan Seventh Day Adventists plan to continue to observe the sabbath on what is now Sunday after Samoa's crossing the date line in December 2011, but the church in Samatau village has decided to adjust and observe the sabbath on the "new" Saturday.[12]
Islam starts and ends Ramadan at new moon, an astronomical phenomenon that occurs at the same instant globally, but at any location will have a time and date appropriate for the local time zone.
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[13][14] 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 the Talmud (Rosh Hashanah and Eruvin) as well as in the Jerusalem Talmud.
The date line poses a problem for religious travellers relative to the day on which to observe the Shabbat and Holidays. The 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 different opinions regarding where exactly the date line is according to Jewish law.[14][15]
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 the 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.[14]
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.[14]
Determining the majority opinion on the Aleutian Islands or South Pacific Islands, including Fiji and American Samoa, is complicated and beyond the scope of this article. However, 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.[14]
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 8:45pm 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 now 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.
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