The year 2000 was the first year of the 2000s decade. Popular culture holds the year 2000 as the first year of the 21st century and 3rd millennium, due to a tendency to group the years according to decimal values, as if year zero were counted. According to the Gregorian calendar, this distinction falls to the year 2001, because the 1st century was retroactively said to start with year AD 1.
Since the calendar has no year zero, its first millennium spans from years 1 to 1000, inclusively, its second millennium from years 1001 to 2000 and its third from 2001 to 3000.
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