All variants are correct. But usually: 2000 - Two thousand; 2015 - Two thousand and fifteen, Two thousand fifteen, and twenty fifteen.

And anything before the 2000's are more commonly read as 1989: nineteen eighty nine 1600: sixteen hundred. (not sixteen thousand) 1900: nineteen hundred Nobody really started saying 'thousand' until the 2000's. I hope this helps.

Although, I don't think anyone actually says "twenty hundred" for the year 2000 People will probably say: 2000: two thousand 2001: two thousand one 2002: two thousand two 2003: two thousand three 2004: two thousand four 2005: two thousand five 2006: two thousand six 2007: two thousand seven 2008: two thousand eight 2009: two thousand nine 2010: twenty ten or two thousand ten 2011: twenty eleven or two thousand eleven 2012: twenty twelve It's a little awkward to say: twenty oh one or twenty oh two

Two thousand = 2000 2010 = two thousand and ten or tenty-ten 2015 =Two thousand and fifteen or twenty fifteen Twenty-ten/ twenty fifteen is easier to say

People say both. If people want to say things quicker, they say "twenty fifteen" for example.