The International Date Line is an imaginary line on Earth's surface defining the boundary between one day and the next. The line is one of convenience because a day has to start and end somewhere.
The 180 degree meridian was selected because it mostly runs through the sparsely populated Central Pacific Ocean. It was decided at the International Meridian Conference in 1884 in Washington, D.C. where 26 countries attended.
The International Date Line is located halfway around the world from the prime meridian about 180 degrees east (or west) of Greenwich, London, UK, the reference point of time zones.
It was first noticed when Ferdinand Magellan and his crew returned from their 16th-century westward circumnavigation of the globe that discovered that one day had somehow been lost.
It runs from the North Pole to the South Pole and marks the divide between the Western and Eastern Hemisphere. It is not straight but zigzags to avoid political and country borders and to not cut some countries in half.
When you cross the International Date Line:
* from west to east - subtract a day
* from east to west - add a day.
Depending on which time zone the country follows, the time difference on either side of the line is not always 24 hours. It is actually possible to have three dates on the planet at one time. Every day between 10:00 and 11:59 UTC, three different dates on the calendar are in use at the same time on Earth. For example:
At 10:30 UTC on May 2, it is...
11.30 pm on May 1 in American Samoa
6:30 am on May 2 in New York
0:30 am on May 3 in Kiritimati
The dateline is not defined by international law and countries are free to choose the date and time zone that they want to observe.
The international boundaries of Russia and the United States has the IDL jutting far to the west to keep the Aleutian Islands together as well as keeping all of Russian territory on the western side of the line. It would be weird to have Russia in two different dates.
Countries near the international date line have moved it over the years to take into account their needs or concerns. One of the biggest zigs occurs around Kiribati, an island nation of 32 atolls that straddles the equator.
When the Republic of Kiribati gained independence from being a British colony in 1979 some of the islands were on one side of the dateline and the rest were on the other. They corrected the anomaly in the eastern half of Kiribati by skipping January 1, 1995 and ever since Kiribati has been the first country to enter the New Year.
It used to be that Somoa and American Somoa were on different sides of the date line, even though they were relatively close together. Then on December, 2011 at midnight, the date line was moved west to group Somoa with its neighbor American Somoa. In the process, the date suddenly became December 31, 2011. They did this to facilitate trade with Australia and New Zealand, and Tokelau followed Samoa for the same reasons.
Three islands that are part of Fiji — Vanua Levu, Rambi and Taveuni — are the only islands actually crossed by the 180-degree line and the line also crosses Wrangel Island, part of the far northeast of Russia and Antarctica.
Samoa officially the Independent State of Samoa and, until July 4, 1997, known as Western Samoa, is a country consisting of two main islands Savaii and Upolu with four smaller islands surrounding the landmasses. The capital city is Apia.
American Samoa is an unincorporated territory of the United States and consists of five main islands and two coral atolls. The largest and most populous island is Tutuila, with the Manuʻa Islands, Rose Atoll, and Swains Island also included in the territory. All islands except for Swains Island are part of the Samoan Islands. They are all "nationals" but not citizens of the United States at birth. Most American Samoans are bilingual and can speak English and Samoan fluently. Samoan is the same language spoken in neighboring independent Samoa.
Nationals have statutory rights to reside in the United States (the 50 states and Puerto Rico), and may apply for citizenship by natualization after three months of residency
While American citizens and nationals can freely move to American Samoa, the American Samoa government controls the migration of aliens to the islands. Special application forms exist for migration to American Samoa based on family-based or employment-based sponsorship.
Unlike other U.S. territories (Guam, Commonwealth of the North Mariana Islands, Puerto Rico, U.S. Virgin Islands), American Samoa is not considered a U.S. State for the purposes of the U.S. Immigration and Nationality Act. This means that lawful permanent residents of the United States may be considered to have abandoned permanent residence if they have moved to live in American Samoa. American Samoa is noted for having the highest rate of military enlistment of any U.S. state or territory.