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

Exact dates

  • Start of the 21st century: 1 January 2001.
  • End of the 21st century: 31 December 2100.

This is because centuries in the Gregorian calendar are counted from year 1 to 100, 101 to 200, and so on, so the 1st century was years 1–100, making the 21st century years 2001–2100.

Why not the year 2000?

Many people celebrated the “new century” on 1 January 2000, and in everyday conversation the 2000s are often thought of as part of the 21st century from that point.

However, from a strict calendar and historical standpoint, the century boundary falls after a full 100-year block is completed, which is why the official rollover was 2001, not 2000.

TL;DR:

  • Official start: 1 January 2001.
  • Official end: 31 December 2100.

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