Everything about The International Date Line totally explained
The
International Date Line (IDL) is an imaginary line on the surface of the
Earth opposite the
Prime Meridian which offsets the date as one travels east or west across it. Roughly along 180°
longitude, with diversions to pass around some territories and
island groups, it mostly corresponds to the
time zone boundary separating +12 and −12 hours
GMT (UT1). Crossing the IDL traveling east results in a day or 24 hours being subtracted, and crossing west results in a day being added.
However, for two hours every day—
Coordinated Universal Time (UTC) 10:00–11:59, there are actually three different days observed at the same time. For example, at UTC time Thursday 10:15, it's Wednesday 23:15 in
Samoa, which is eleven hours behind UTC, and it's Friday 00:15 in
Kiritimati, which is fourteen hours ahead of UTC. For the first hour (UTC 10:00–10:59), this phenomenon affects inhabited territories whereas during the second hour (UTC 11:00–11:59) it only affects an uninhabited maritime time zone twelve hours behind UTC.
The first date-line problem occurred in association with the circumnavigation of the globe by
Magellan's expedition (1519–1522). The surviving crew returned to a
Spanish stopover sure of the day of the week, as attested by various carefully maintained sailing logs. Nevertheless, those on land insisted the day was different. Although now readily understandable, this phenomenon caused great excitement at the time, to the extent that a special delegation was sent to the
Pope to explain this temporal oddity to him.
Geography
For a small part, the International Date Line follows the
meridian of 180° longitude, roughly down the middle of the
Pacific Ocean. However, because the date to the east of the line is one day earlier than that to the west of the line, the line deviates to pass around the far east of
Russia and various island groups in the
Pacific. Thus, the two largest deviations from this meridian both occur to keep the date line from crossing nations internally.
In the north, the date line swings to the east through the
Bering Strait and then west past the
Aleutian Islands in order to keep
Alaska (part of the
United States) and Russia, which are due north and south of each other in that region, on opposite sides of the line and in agreement with the date of the rest of those countries. The date line passes equidistant between the two
Diomede Islands, Little Diomede Island (US) and Big Diomede Island (Russia), at a distance of 1.5km (1mi) from each island.
The date line circumvents the territory of
Kiribati by swinging far to the east, almost reaching the 150° meridian.
In the South Pacific, the dateline swings east such that
Wallis and Futuna,
Fiji,
Tonga, and
New Zealand's
Kermadec Islands have the same date, but
Samoa is one day earlier.
The International Date Line can cause confusion among airline travelers. The most troublesome situation usually occurs with short journeys from west to east. For example, to travel from
Tonga to
Samoa by air takes approximately two hours, but involves crossing the International Date Line, causing the passenger to arrive the day before
s/he left. This often causes confusion in travel schedules (unless those schedules quote times in
UTC, but they don't since they must match domestic travel times, local transport or meeting times).
If someone circumnavigates the globe in an airplane from east to west (the same direction as
Magellan), s/he should subtract one hour for every 15° of longitude crossed, losing 24 hours for one circuit of the globe. But 24 hours are added when crossing the International Date Line (from east to west). The International Date Line must therefore be observed in conjunction with earth's time zones: the net adjustment to one's watch is zero. If one crosses the date line at precisely midnight, going westward, one skips an entire day; while going eastward, one repeats the entire day.
Cultural references
The effect of ignoring the date line is also seen in
Jules Verne's book
Around the World in Eighty Days, in which the travelers, led by
Phileas Fogg, return to London after a trip around the world, thinking that they've lost the bet that's the central premise of the story. Having travelled the direction opposite that taken by Magellan, they believe the date there to be one day later than it truly is.
Lest anyone accuse Fogg of cheating by obtaining one extra day, this isn't so. Assuming a constant eastward speed, each travel day was 18 minutes short of a full 24 hours, accumulating to one full day, which they failed to correct as we'd by setting our calendar back a day in mid-Pacific. Also, similar information is found in "
The Twenty-One Balloons".
Anyone traveling
west and passing the line must add a day to what they'd otherwise expect the date and time to be. Correspondingly, those going
east must subtract a day. Magellan's crew and Verne's travelers neglected to make those respective adjustments.
The date line is also a central factor in
Umberto Eco's book
The Island of the Day Before, 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.
De facto and de jure date lines
The IDL drawn on the map on this page and all other maps is now and always has been an artificial construct of cartographers—it is
de facto. No international organization nor any treaty between nations has fixed the 'straight line' segments and their junctions. All nations unilaterally determine their standard
time zones, which are applicable only on land and adjacent territorial waters. These national zones don't extend into international waters. Indeed, the 1884
International Meridian Conference explicitly refused to propose or agree to any time zones, despite 'common knowledge' that they did, stating that they were outside its purview. The conference resolved that the Universal Day (midnight-to-midnight
Greenwich Mean Time), which it did agree to, "shall not interfere with the use of local or standard time where desirable."
The
nautical 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 US 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's interrupted by territorial waters adjacent to land, forming gaps—it is a pole-to-pole dashed line. Ships must adopt the standard time of a country if they're within its territorial waters, but must revert to international time zones (15° wide pole-to-pole gores) as soon as they leave its territorial waters. In reality they use these time zones only for radio communication etc. Internally in the ship, for example for work and meal hours they use a suitable time zone of their own wish. The 15° gore that's offset from GMT or UT1 (not
UTC) by twelve hours is bisected by the nautical date line into two 7.5° gores that differ from GMT by ±12 hours.
Historical alterations
The
Philippines, as part of the
Viceroyalty of 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.
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. The day before the
purchase by the
United States took effect, it was Friday,
6 October 1867, in the
Julian calendar (used by Russia at the time), which would have been
18 October in the
Gregorian calendar. The time in
New Archangel would have been 12:00 when it was 12:02, Thursday,
17 October, at the future site of
Whitehorse, Yukon and 12:49,
17 October, at the future site of
Vancouver, British Columbia. With the transfer of governance, the date line was shifted (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 11 days. Friday,
6 October was followed by Friday,
18 October, not Saturday,
7 October.
Samoa changed in 1892, eight years following the international conference that would result in
de facto development of the Date Line. The
king 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 at the end of the day, Monday,
4 July 1892, so that year, there were 367 days, including two occurrences of Monday,
July 4.
The central Pacific Republic of
Kiribati introduced a change of date for its eastern half, on
January 1,
1995, from time zones −11 and −10 to +13 and +14. Before this, the country was divided by the date line. This meant that 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 the
Phoenix and
Line Islands from the United States and the country found itself straddling the date line. Government offices on opposite sides of the line could only communicate on the four days of the week when both sides experienced weekdays simultaneously. A consequence of this time zone revision was that Kiribati, by virtue of its easternmost possession, uninhabited
Caroline Atoll at 150°25′ west, started the year 2000 on its territory before any other country on earth, a feature which the Kiribati government capitalized upon as a potential tourist draw. But Ariel and Berger comment that the international community have not taken this date line adjustment seriously, noting that most world atlases still ignore this Kiribati dateline shift and continue to represent the International Date as a straight line in the Kiribati area.
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