30N,180E International Date Line
Ferdinand Magellan, the captain general of the Armada de Molucca of 5 ships and 240 men, led a 3-year voyage around the world, beginning in 1519, the first circumnavigation of the globe. He was hacked to death by angry villagers in the bay of Macatan in the Philippines, so his eventual replacement Juan Sebastian Elcano, sailed into Seville, Spain with a boat full of cloves and only 18 survivors. They discovered, after keeping a written day-by-day account of the entire voyage, that they had gained a day.
In 1521 Antonia Pigafetta, the diarist of the expedition, remarked, “We were greatly surprised for it was Wednesday with us, and we could not see how we had made a mistake; for I had always kept well, and had always set down every day without interruption.” Not Magellan, or any other astronomer, astrologer, cartographer, or even Ptolemy himself, had considered that if you sail around the earth chasing the sun West, you will see one less sunset and sunrise, and return to the point you started with an extra day, but not extra time.
The opposite happens if you sail east around the world. Your days are minutely shorter because you are racing toward the rising sun, and therefore experience sunrise earlier than the people you left at the dock. So by the time you get back to where you started you’ve lost a day. To correct this shift in calendar time we have the International Date Line, which we have crossed today.
At midnight on June 23 we crossed 180, only to start June 23rd all over again.
Reference: Over the Edge of the World: Magellan’s Terrifying Circumnavigation of the Globe (2003), Laurence Bergreen.