All the years in this century start with 20… So how come it’s called the 21st Century?
Well, first of all, not all the years start with 20… The very last year of this century, its 100th year, will be 2100.
It works just like your age. Are you 12 years old? That means you’re in your 13th year. When you’re in your 21st year, you’ll be 20 years old until the very last day—the day before your birthday. Then on that birthday you’ll be 21 years old and in your 22nd year.
Anno Domini is in its 21st century, and is 2019 years old. December 31, 2100 will be the last day of the 21st Century. On January 1, 2101, AD will be in its 22nd century.