A year is a leap year if (divisible by 4 AND not divisible by 100) OR divisible by 400
2000: divisible by 400 → leap year. 1900: divisible by 100 but not 400 → not a leap year.
Century years must be divisible by 400 to be leap years; 1900 fails that test while 2000 passes.
366, with the extra day added as February 29.