Why the Years in the 2000s Are Called the 21st Century?
It appears that we are a century ahead of our calendar because we refer to the years in the 2000s as the 21st century, not the 20th century. A century is a period of 100 years. We refer to the years 1 through 100 A.D. as the first century. The second century ran from 101 through 200.
Referring to the latter as the second century is what seems to put us one century ahead of our time, for in the year 201, we entered the third century. Counting in this manner, we find that the current century—the years between 2000 and 2100—are indeed the 21st century.
The 21st century is the current century of the Anno Domini era or the Common Era, in accordance with the Gregorian calendar. It began on January 1, 2001, and will end on December 31, 2100. It is the first century of the 3rd millennium. It is distinct from the century known as the 2000s, which began on January 1, 2000 and will end December 31, 2099.