２０００年代にせんねんだい

noun

the 2000s (decade 2000-2009); the '00s; the aughts; the noughties

Refers specifically to the ten-year period from 2000 to 2009. In Japanese, 年代 usually means a decade.

• ２０００年ねん代だいの音おん楽がくは今いまでも人にん気きがある。

  Music from the 2000s is still popular today.

noun

the 2000s (century 2000-2099)

Can refer to the entire 21st century when context makes it clear, though ２１世紀 is more common for the century.

• この技ぎ術じゅつは２０００年ねん代だいを通つうじて発はっ展てんするだろう。

  This technology will develop throughout the 2000s.

noun

the 2000s (millennium 2000-2999)

Rarely used for the entire millennium; usually ２０００年代 means the decade. This sense appears mainly in futuristic or historical contexts.

• 遠とおい未み来らいの２０００年ねん代だいには、人じん類るいは宇う宙ちゅうに進しん出しゅつしているかもしれない。

  In the distant 2000s, humanity may have expanded into space.