The day from your birth up to your first birthday is your first year on earth.
So [0, 1) is your first year.
[0, 10) is your first decade
[0, 100) is your first century
[100, 200) your second century
…
[2000, 2100) your twenty-first century
Now apply this to a calendar instead of your birthday and you have your answer:
The number marks the years that have – passed – while the term “21st century” marks the time span that is – going on currently.
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