I thought century years couldn't be...

Yes. A year is a leap year if it is divisible by 4, but century years are not leap years unless they are divisible by 400. So, the years 1700, 1800, and 1900 were not leap years, but the year 2000, as of course you now know, is! Why do we have leap years? The actual length of a year (the rotation of Earth around the Sun) is 365.2422 days. If we didn’t have leap years, the seasons would shift about a quarter of a day every year, and after 100 years the seasons would be off by 25 days. The extra leap day adjusts this drift.