Enter any year, and the tool tells you instantly whether it's a leap year, plus the next and previous leap years.
A year is a leap year if it is divisible by 4, except for century years, which must also be divisible by 400. So 2000 was a leap year but 1900 was not.
Earth takes about 365.2422 days to orbit the Sun. Adding a day every four years keeps the calendar aligned with the seasons; the century rule corrects the small remaining error.
No. 2100 is divisible by 4 but it is a century year not divisible by 400, so it is a common year of 365 days.