The 21st century officially started on 1 January 2001 and will end on 31 December 2100.

Why it starts in 2001 (not 2000)

  • The calendar starts at year AD 1, not year 0.
  • That means:
    • 1st century: years 1–100.
* 2nd century: years 101–200, and so on.
  • By this logic, the 20th century runs from 1901–2000, so the 21st century runs from 2001–2100.

But wasn’t 2000 celebrated as the “new century”?

Many people and media treated 1 January 2000 as the start of the 21st century because the year number changed from 19xx to 20xx, which felt like a big symbolic shift.

So:

  • Technically correct (astronomers, historians, official bodies):
    • 21st century: 2001–2100.
  • Popular/cultural usage:
    • Many people casually think “years starting with 20” = 21st century, so they count from 2000.

Quick forum-style recap

Q: When did the 21st century start?
A: Technically, 1 January 2001; culturally, lots of people treated 1 January 2000 as the “start” because the year flipped to 2000.

TL;DR:

  • Exact calendar definition: 21st century = 1 January 2001 to 31 December 2100.
  • Everyday talk: people often lump 2000 in with the 21st century because it begins with “20.”

Information gathered from public forums or data available on the internet and portrayed here.