Question: how do you calculate century years?
Answer: Add 1 to the first 2 digits (on the left) of the year of birth, unless it is a year that ends in 00.
Example: 1999 => 19 + 1 = 20 => 20th century.
Example: 2000 => 20 => 20th century.
Example: 2001 => 20 + 1 = 21 => 21st century.