The International date line runs through the Bering Strait, drops down through the Pacific Ocean to the east of New Zealand and ends by sliding across icy Antarctica. Travelers who cross the Pacific Ocean going west have one day added to their journeys. Travelers who cross the ocean going east have one day subtracted. Changing dates doesn't require going entirely across the ocean, merely crossing the IDL. Travel a little over 3,000 miles southwest from Hawaii to Fiji and the date will move forward by one day.
Talmudic texts from the late 12th century refer to the need for an international date line. Syrian and French scholars addressed the "circumnavigator's paradox" in the 13th century: Three people meet at the same place along the equator; one travels around the globe to the east; one around the globe to the west; the third remains in the place where the three met. The two travelers return to the spot where the third waits, and each believes a different number of days has elapsed.
Each traveler in the "circumnavigator's paradox" measured the journey by the movement of the sun, as did the person who remained at the starting point. The sun's position and movement were different depending upon the direction each person in the paradox traveled; the person who didn't move also experienced the rising and setting of the sun differently. Thus, each person calculated the excursion in different ways. The person who traveled west had the shortest trip, while the one who traveled east had the longest trip.
Once travel around the globe became a reality, the theoretical date problems became realities as well. Spanish explorer Ferdinand Magellan encountered the problem first when he successfully sailed from the Atlantic to the Pacific. Countries attempted to solve the dilemmas caused by global circumnavigation as early as the 15th century; these solutions were largely dictated by trade routes in the Pacific Ocean. In the early 17th century, the Philippines adopted a Day of Reckoning to resolve the issue of differing travel times.
The Prime Meridian was agreed upon in 1884 by delegates to the International Meridian Conference; it established a line running through Greenwich Park, London, England, as 0 degrees longitude. The adoption of this location for the Prime Meridian meant that the logical course for the IDL was 180 degrees to the west. Conveniently, that location cut mainly through open ocean rather than land masses. Still, there has never been a formal agreement delineating the international date line; it has evolved over time through territorial and commercial imperatives.
