Why a westward voyage fell behind the calendar
The crew did not make their days pass more slowly. They carried a calendar tied to the ship, while the date at any place on land followed that place’s cycle of daylight and local time. As the vessel sailed west, the Sun’s apparent position shifted later relative to the ship: sunset came later along the route, so the crew continually encountered local times earlier than those they had left behind.
The difference accumulates with longitude. Earth turns 360 degrees in about 24 hours, so moving roughly 15 degrees west corresponds to a one-hour shift in local solar time. Over a journey around the world, those small shifts add up to a full day. If the crew adjusted its sense of time to each new locality, westward travel effectively stretched the ship’s calendar day; if it kept counting days aboard, that count no longer matched the date at home.
A missing date, not a lost day at sea
The mismatch became apparent when the surviving crew returned from the voyage in 1522 and found that their records were one calendar day behind the date ashore. No day had vanished from their lives. They had experienced the ordinary succession of sunrises and sunsets, but had counted them while traveling through changing local times.
A fixed international date line now provides a shared place to change the calendar by one day when crossing the Pacific. The crew had no such agreed line to consult. Their discrepancy was the bookkeeping consequence of carrying a westward day-count all the way around the globe.
