When was international date line established?

Answer 1

The very quick answer: It was never actually established by any kind of international law or agreement.

The history link below is particularly helpful, if rather detailed. First mention of the 'circumnavigator's paradox', the loss or gain of one day for those travelling around the globe, is from a Syrian prince and historian during the late 1200's or early 1300's. Later geographers and philosophers made reference to it. With Magellan's (1480-1521) historic circumnavigation, the paradox was at last experienced.

Since Magellan there have been a few different ways to solve the problem, as the history shows. As far as I can determine, there has never been set by any international agreement the actual location of what we now call the International Date Line; it came about essentially through convention and common use. Any nation is free to establish whatever standard time it sees fit, and this includes dates.

The International Date Line is theoretically the line of 180 degrees longitude, the meridian exactly opposite the Prime Meridian or Greenwich Meridian. From the beginning, various nations on or near to the 180 degree meridian have established that they are on one side or the other of the 'date line', and its 'location' is always subject to change.

The location of the date line is independent of the time zone(s) that span a nation or territory. The easternmost region of the Pacific island nation of Kiribati, Caroline Island, experiences midnight as it always has relative to locations to its north and south. but since 1995 Kiribati declared that midnight at Caroline Island marks the beginning of the day to the west of the International Date Line; Kiribati 'moved' the line to its east.

If you and a friend are standing face to face with the IDL between you, the one to the west is one day advanced compared to the one to the east. It would have been less drastic for Kiribati to 'move' the dateline to its west instead, leaving all of Kiribati on the same side of the line as North and South America. Some have argued that the arbitrary move was to foster tourism, since Caroline Island became, by definition, the first location on earth (along with Antarctica, of course) to experience the change of millennium.

The Prime Meridian was internationally decided by the International Meridian Conference of 1884, but that conference did not presume to fix or decree the location of the date line or boundaries of time zones, wisely concluding that these were matters for sovereign nations to decide for themselves. In fact the location of a prime meridian and of a date line, while very important, are entirely arbitrary, and have nothing whatever to do with any special characteristic or property of any part of the globe.