One century equals 100 years. And it depends. The 21st century is of 36,503 days. But the 20th had 36,504 days. What's the difference?. If you wanna know Check out the Gregorian Calendar and try to understand it deeply and you will understand everything about the time. It's not such a big number when you think about it but of course, a century passes inexorably. Some of us complete the journey in a bit less than 3.16 billion seconds.