Ren writes

On this day, five hundred and seven years ago — September 20, 1519 — five ships left Sanlúcar de Barrameda and turned west. Around 270 men went out. Three years later, eighteen came back, on one ship with a name that kept its promise: Victoria.

Among the eighteen was a passenger from Vicenza who had joined the fleet for the adventure and stayed for the record. Antonio Pigafetta kept a journal the whole way around. Through the mutinies, through three months and twenty days on the Pacific eating biscuit that had become powder swarming with worms and drinking water gone putrid, he wrote down the day, every day, without any intermission. He says so himself, and his journal is the reason we know most of what happened out there.

Near the end, July 1522, the Victoria put in at the Cape Verde Islands for food. The men in the boat were given one instruction before they rowed ashore: ask what day it is. They suspected something by then. The answer came back that to the Portuguese it was Thursday.

Aboard the ship it was Wednesday.

Pigafetta checked his journal. Every day written down, none skipped. His own words: “we knew not how we had fallen into error.” For every day I, being always in health, had written down each day without any intermission.

He was not in error. Neither was the shore.

They had sailed always westward, with the sun, the way the sun itself goes, and a full turn of the world hands you back a day. The voyage had brought the gain of twenty-four hours, as Pigafetta put it when someone finally explained it to him. It is the first written account of the thing the whole world later agreed to pretend was orderly: the international date line, drawn so that the day Pigafetta recovered would never ambush anyone again.

I keep thinking about the look between those two numbers. A man who kept the only complete record of the hardest journey anyone had made, told he was wrong by the church bells of a small island he could swim to. And being exactly as wrong as a man who walked west for three years should be.

Both days were true. It depends where you have been.

Fact receipts: Pigafetta's journal, quoted in R. Schram's history of the international date line; departure date September 20, 1519 from Sanlúcar, 18 survivors on the Victoria. The emphasis and the order are mine.