The International Date Line (IDL) is an imaginary line on the surface of the
Earth opposite the Prime Meridian where the date changes 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 Coordinated Universal Time (UTC)
(Greenwich Mean Time – GMT). Crossing the IDL travelling east results in a
day or approximately 24 hours being subtracted, and crossing west results in
a day being added. The exact number of hours depends on the time zones.
Geography
For part of its length, the International Date Line follows the meridian of
180° longitude, roughly down the middle of the Pacific Ocean. In order to
keep from crossing nations internally, however, the line deviates to pass
around the far east of Russia and various island groups in the Pacific.
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 in
the rest of those countries. As a result of this line-adjusting, the right
to call itself "The Last Place on Earth" (that is, the latest place) goes to
the westernmost Aleutian Island of Attu.
The date line passes equidistantly between the two Diomede Islands—Little
Diomede Island (US) and Big Diomede Island (Russia)—at a distance of 1.5 km
(1 mi) 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 date line 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. To travel from Tonga to Samoa by air, for example, takes approximately
two hours but involves crossing the International Date Line, causing
passengers to arrive the day before they left. This often causes confusion
in travel schedules, like hotel bookings (unless those schedules quote times
in UTC, but they typically do not since they must match domestic travel
times, local transport, or meeting times). Some examples of time zone
adjustments for real air trips are: Alaska-Siberia 21 hours, New
Zealand-Cook Islands 22 hours, and Samoa-Tonga 24 hours.
If someone circumnavigates the globe in an airplane from east to west (the
same direction as Magellan), they 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.
For two hours every day, at UTC 10:00–11:59, there are actually three
different days observed at the same time. At UTC time Thursday 10:15, for
example, it is Wednesday 23:15 in Samoa, which is eleven hours behind UTC,
and it is Friday 00:15 in Kiritimati (separated from Samoa by the IDL),
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.
Cultural references
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. This phenomenon, now readily
understandable, 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.
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 have
lost the bet that is the central premise of the story. Having traveled the
direction opposite to the one 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 is not
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 would by setting our calendar back a day in mid-Pacific. Also,
similar information is found in The Twenty-One Balloons.
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 do not extend into
international waters. 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), 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 is 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 are
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, e.g. for work and meal hours they use a suitable
time zone of their own wish. 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.
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 on Monday, 4 July 1892,
so there were 367 days (1892 being a leap year), including two occurrences
of Monday, 4 July.
The central Pacific Republic of Kiribati introduced a change of date for its
eastern half on 1 January 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 by radio or telephone 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, the 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 has not taken this date
line adjustment very seriously, noting that most world atlases still ignore
this Kiribati dateline shift and they continue to represent the
International Date as a straight line in the Kiribati area.
Jewish date lines
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 Hashana, the Jewish New Year. According to some
sources it is alluded to in the Talmud (Rosh Hashana and Eruvin) as well as
in the Jerusalem Talmud.
The date line poses a problem for religious travelers 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 you stay on the same side of the date line. The problem occurs when a Jewish traveler crosses the line and for him it is Friday but for the city he is visiting, it is Saturday. Most authorities[by whom?] hold that he should keep his own calendar (so it is still Friday for him) until he meets locals for whom it is Saturday and only then would he use the local calendar.
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