Why do people think centuries end one year before they do?

[Conina]

barneystinson69 said:

RolStoppable said:
Hold on a second. How can the first part be correct? When a new century starts in 2001, then a new millenium also starts in 2001. After all, a millenium is exactly ten centuries, so it has to align with them.

https://en.wikipedia.org/wiki/Millennium

Ok, so now I'm confused. Some say it should be celebrating in 2000, others say 2001.

If the Gregorian calendar had startet at zero (January 1st, year 0), then January 1st, year 1000 and 2000 would have been the correct dates for the millennia.

But unfortunately the Gregorian calendar had startet at one (January 1st, year 1). Therefore January 1st, year 1001 and 2001 would have been the correct dates for the millennia.