A century means one hundred years
The first century covered years 1–100, so the 21st century covers 2001–2100. There was no year zero in this numbering convention. A century is always 100 years, but the number of days depends on which leap years it includes.
A calendar example
January 1, 2000 to January 1, 2100 spans 36,525 calendar days, while January 1, 1900 to January 1, 2000 spans 36,524. Gregorian years divisible by 100 are leap years only when they are also divisible by 400.