What is the International Date Line?
The international date line is a boundary that separates two days. Strictly speaking if flying at the correct speed one can fly around the world without ever changing the apparent time since the earth spins in relation to the sun. Because of the rotating earth the world is divided into time zones so that when, for instance, some mentions 5 PM the experience is the same for everyone rather than some people having evening at 5 PM with others having early morning, etc. The international dateline is where these time zones change days, so on either side of the date line the date is always different.
Where is the International Date Line?
The International Date Line is in the pacific ocean. So as to not divide any nation the line bends as it moves from the boundary between the Russian Federation and the United States into the Middle of the Pacific where Hawaii lies west of Russia's most eastern border to the north.
How is The International Date Line Used?
No international agreement has ever extended time zones beyond an invidiual nation so the actual boundaries of the line are somewhat subjective to the nations involved. In fact even at Sea, where international agreements to keep time zones have been made, the use is inconsistent. That said wherever nations decide to draw the line, its use is fairly accepted. On either side the time is either 12 hours ahead of Greewich Mean Time or 12 hours behind it. So for instance if it were 12 noon on August 25th on the eastern side of the dateline it would be 12 noon on August 26th on the other. This is confusing for short trips since it results in apparently having been to the future.
Discovery of the International Date Line?
The International Date Line was discovered when Magellan first traveled around the world and confusion arose when the ship's crew landed, convinced the day was different than that stated by those on land. The book around the world in 80 days also mentions this problem in the context of the books plot. The main characters believe they've lost a bet due to their at sea calculation of the time.
How has the International Date Line changed through the years?
Before Alaska was purchased by the United States the International Date Line crossed land at the easternmost boundary of the Russian Empire. When the United States purchased this, it unilateraly changed the date to the Gregorian calendar and the corrected Gregorian time. This plunged Alaska a few minutes over 12 days into the future since the Julian calendar is off by almost 2 weeks from the Gregorian. Also, a change in Samoa during leap year resulted in a year with 367 days.