The first year ended at the end of year 1.
The first decade ended at the end of year 10.
The first century ended at the end of year 100.
The first millennium ended at the end of year 1000.
It takes 10 complete years to have a decade, 100 complete years to have a century, and 1000 complete years to have a millennium.
It's not rocket science. It's mathematics.
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