Portuguese explorer Ferdinand Magellan is credited with leading the first circumnavigation of the globe, but his expedition also deserves credit for discovering the need for an International Date Line.

When the 18 remaining crew members of Magellan’s expedition reached the Cape Verde islands in July 1522, they realized their calendars were behind a day, even though they kept impeccable records in the ship’s logs. The discovery led to a special delegation explaining the oddity to the pope.

Although the concept had been theorized as early as the 14th century, the International Date Line wasn’t officially proposed until the International Meridian Conference in 1884.