It's a date
Here's some teasers to get you to read a short but interesting history of the "phenomenon" in:
The Border That Stole 500 Birthdays
The date line is the logical consequence of the so-called Circumnavigator’s Paradox, which was known to scientists before it was witnessed for the first time by Antonio Pigafetta in the early 16th century.
But there is no reason it should run across the Pacific, except the convenience of avoiding populated areas. As it happens, its course was determined more or less by default. The I.D.L. is a byproduct of the International Meridian Conference of 1844.
but neither the International Meridian Conference, nor any other subsequent global committee, ever sanctioned its “official” use. As the English-American geographer George Davidson said at the end of the 19th century: “There is no International Date Line; the theoretical line is 180 degrees from Greenwich, but the line actually used is the result of agreement among the commercial steamships of the principal maritime countries.”
The International Date Line may not be an officially sanctioned border, and its raison d’être rather counter-intuitive, but it is essential for good, global time-keeping. Its unofficial nature allows its bordering nations to switch sides according to political and economic expedience. And its genesis as an afterthought to the Prime Meridian recalls a time when Europe was at the center of the world. In fact, Greenwich and the International Date Line can be seen as carbon copies to the meridians of Tordesillas and Zaragoza, which divided the world between Spain and Portugal in the 16th century.
Pigafetta also penned the first European observations of Guambat's little island burrow, back in 1521:
[W]e discovered, on Wednesday, 6 March a small island to the northwest, and two others toward the southwest, one of which was higher and larger than the other two. The captain-general [Magellan] wished to stop at the large island and get some fresh food, but he was unable to do so because the inhabitants of that island entered the ships and stole whatever they could lay their hands on, so that we could not protect ourselves. The men were about to strike their sails so that we could go ashore, but the natives very deftly stole from us the small boat that was fastened to the poop of the flagship. Thereupon, the captain-general in wrath went ashore with forty armed men, who burned some forty or fifty houses together with many boats, and killed seven men. He recovered the small boat, and we departed immediately....So what was that infamous day in March 1521?
Each one of these people lives lives according to his own will, for they have no Seignior. They go naked, and some are bearded and have black hair that reaches to the waist. They wear small palm leaf hats, as do the Albanians. They are as tall as we, and well built. They have no worship. They are tawny, but are born white. Their teeth are red and black, for they think that is most beautiful. The women go naked except that they wear a narrow strip of bark as thin a paper which grows between the tree and the bark of the palm, before their privies. They are good-looking and delicately formed, and lighter complexioned than the men, and we their hair, which is exceedingly black, loose and hanging quite down to the ground.
Their amusement, men and women, is to plough the seas with those small boats of theirs. Tese boats resemble fucelere, but are narrower, and some are black, some white, others red. At the side opposite the sail, they have a large piece of wood pointed at the top, with poles laid across it and resting on the water, in order that the boats may sail more safely. The sail is made from palm leaves sewn together and is shaped like a lateen sail. For rudders, they use a certain blade resembling a hearth shovel that has a piece of wood at the end. They can change stern and bow at will, and those boats resemble the dolphins that leap in the water from wave to wave. Those Ladroni thought, according to the signs which they made, that there were no other people in the world but themselves.
[From the translation provided in The Chamorros of the Mariana Islands, Early European Records, 1521 - 1721, Glynn Barratt, Occasional Historical Papers Series, No. 10, CNMI Division of Historic Preservation, 2003.]