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The millennium is a unit of time equal to 1,000 years. A millennium is used for specific periods of one thousand years that begin at initial reference point of the calendar in consideration; however, a millennium may also refer to any period of 1,000 years long.
0.005 millennia correspond to one lustrum (5 years).
0.001 millennia correspond to one decade (10 years).
0.1 millennia correspond to one century (100 years).
In the Gregorian Calendar (that has no year zero), the millennia is counted from 1 AD:
- First millennium: year 1AD - year 1000
- Second millennium: year 1001 - year 2000
- Third millennium: year 2001 - year 3000
In the astronomical numbering system (that assumes a preceding Year 0 for the start of the first Christian millennium):
- First millennium: year 0 - year 999
- Second millennium: year 1000 - year 1999
- Third millennium: year 2000 - year 2999