Many people assume every year divisible by 4 is a leap year. However, the year 1900 was NOT a leap year, and the year 2100 will NOT be a leap year either!
The Astronomical Mathematical Proof
Because the solar year is 365.2422 days rather than 365.25 days, a simple 4-year leap cycle adds 0.0078 excess days per year. Over 400 years, this accumulates to exactly 3.12 excess days. Dropping 3 leap years every 400 years (years divisible by 100 but not 400) keeps the calendar accurate to within 1 day every 3,236 years!
Learn more on our Leap Year Guide.