The 90’s is a decade and went from January 1st, 1990 to December 31st 1999.

Century’s are off by 1 year because there’s no year “0”, so every Century starts with “1”. 1st century is 1-100, then 101-200, 201-300, 301-400, etc.

That’s also why we’re in the 21st century right now, instead of the 20th century, despite the years starting with 20__.