Leap Day — The Intercalary Mathematics

Leap Day occurs approximately every four years, but the rule is more precise: a year is a leap year if divisible by 4, except century years, which must be divisible by 400. So 1900 was not a leap year, but 2000 was. This correction — adding 97 leap years per 400 years — keeps the calendar accurate to within one day per 3,030 years. The probability of being born on February 29 is about 1 in 1,461. In Gilbert and Sullivan's The Pirates of Penzance, Frederic is apprenticed until his 21st birthday — but since he was born on Leap Day, this turns out to be his 84th year.