Century

Because the first century started at zero.

Actually, the first century started in the year one, but the point is still the same. The first year of the first century was one, and the last year of the first century was 100. The first year of the 21st century was 2001, and the last year of the 21st century will be 2100.

We aren't in the year 2013 yet. A little thought will tell you that the first century started in the the year 1 and lasted 100 years. The second century started in the year 101 AD, the third century started in the year 201 and so on. I know it is the fashion nowadays to say a century starts in the 00 year but there was no year 0 AD/BC.

Ahhh...those were the good ol' days