The earliest records of the Celendrical (international) date line, the point upon the Earth where today inextricably becomes tomorrow, are found in the bemused writings of perplexed explorers from the 16th century. An expedition led by Magellan arrived in the Cape Verde islands on, to them, a Wednesday, only to find that on the islands, it was Thursday. Similarly, an expedition led by Drake around the world, arrived home on what they considered to be Sunday, to find that it was in fact Monday in Britain! To unravel this conundrum, we must first consider the nature of time and what it is that we measure using a calendar.
Since, from where we stand on Earth, the Sun rises in the East and sets in the West, it follows that the dawn of a new day sweeps across the Earth in the same fashion. It also follows that were we able to halt the apparent progression of the Sun, it would appear to be dawn in one place, noon in another and dusk in a third. More accurately, of course, since the Earth rotates on its axis each day so that different areas are bathed in sunlight at any one time, we would need to halt the rotation of the Earth to achieve the same effect. Either way, if time is reckoned by the position of the Earth relative to the Sun, it is noon at different moments across the Earth and it will always be noon in the Middle East before it is noon in Florida. Somewhere, it will be today; across the world, it will be nearly tomorrow. It cannot be otherwise.
In a world where we are ruled by knowing exactitudes, it is not enough to know that a thing IS, rather, that thing must be understood and categorised and measured. Humanity has attempted to measure and to categorise time since 4241 BC, the first known date from Ancient Egypt. Various calendars attempting to make sense of solar time and human perception of days have been in existence for millennia, with the Gregorian calendar finally being universally accepted in 1949 AD.
All systems of measurement require a reference point and the measurement of time is no exception. In this case, the universal reference points are the Greenwich Meridian and its corollary line, the Celendrical date line. Prior to the definition of Greenwich as the Prime Meridian, the line relative to which all other times are calculated, individuals reckoned the exact and true time to be essentially that time decreed in the major settlement in the region. Thus, the French calculated the time with reference to Paris and the British with reference to London (Greenwich).
By 1884 the Greenwich Meridian had been accepted as the Prime Meridian. Once this had occurred, working out the exact official time in any other locality was simply a matter of calculation. If, in Greenwich, it is noon, with the sun directly overhead, then on the other side of the Earth, it must be midnight and the twelve hours in between can be divided up. The division of the day (one rotation of the Earth upon its axis) into twenty-four hours dates from Babylonian times and is probably related to the Zodiac – – an intriguing story in and of itself, but one that will not be addressed here. The globe can therefore be divided into twenty-four time zones, all a set number of hours different from Greenwich. Thus, travelling east from Greenwich, there will come a point that is twelve hours ahead of Greenwich Time; midnight of today and the start of tomorrow. But travelling west from Greenwich, the same point in space, twelve hours later than Greenwich Time, is definitely midnight, the start of today. This point, both today and tomorrow at the same time, is the International or Celendrical Date Line.