9 BC – 1 BC and 1–9 AD are technically considered decades in either the proleptic Gregorian calendar or Julian calendar even if they only lasted for nine years each.
following 1–9 AD is the 10s decade, which ran from January 1, 10, to December 31, 19. It was the second decade in the Anno Domini/Common Era.
The 20s decade ran from January 1, 20, to December 31, 29.
The 2000s is the decade that started on January 1, 2000 and will end on December 31, 2009. It is the current decade.
Thus, 1990 is part of the 90s decade and not the 80s.
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