Why is it called the 21st century and not the 20th?

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Tags

1. #domini
2. #known
3. #which
4. #first
5. #january
6. #accordance
7. #gregorian
8. #2000s
9. #began
10. #distinct
11. #common
12. #twenty
13. #december
14. #current
15. #calendar
16. #century