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

[VGPolyglot]

Conina said:

barneystinson69 said:

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.

The problem is also that the Gregorian calendar actually started in 1582, which changed the Julian calendar. According to this site, apparently the Julian calendar was actually introduced in 45BC, so 1001 would not be the start of a new millennium.

https://www.timeanddate.com/calendar/julian-calendar.html