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Picture going to bed on Thursday, October 4, 1582, and waking up on Friday, October 15. No hangover, no coma, no time machine involved. The ten days in between were simply gone.
The culprit was a tiny math error. The old Julian calendar overestimated the solar year by about 11 minutes and 14 seconds, which seems harmless until it piles up for over a thousand years.
By the mid-1500s, the spring equinox was landing around March 11 instead of March 21. That mattered because the Council of Nicaea in 325 had tied the date of Easter to a March 21 equinox.
Pope Gregory XIII issued the bull Inter gravissimas on February 24, 1582. His fix was blunt: delete ten days from the calendar in one go, so the next date after October 4 would be October 15.
It wasn't the only option. Astronomer Aloysius Lilius had proposed a gentler plan, dropping the Julian leap day ten times over 40 years, but the bull chose the faster route.
To stop the drift from returning, the new rule changed leap years. Century years only count if divisible by 400, so 1600 and 2000 were leap years, but 1900 was not, giving an average year of 365.2425 days.
Not everyone went along. Britain waited until 1752 and skipped 11 days, Russia held out until 1918, and Greece adopted it for civil use in 1923. For centuries, the date depended on which side of a border you stood.
