What does the fourth millennium BC or BCE mean in terms of beginning and end dates?

Answer 1

A millenium is 1000 years long from beginning to end. In terms of the modern calendar, there is no year-long period called year zero, no century zero and no millenium zero. As it is on the standard number line, 'zero' is a point in time on the calendar.

So, the year 1 BCE (Before Common Era, or BC for Before Christ) ended on December 31 1 BCE, the day before January 1, 1 AD. That first year started on the previous January 1, 1 BCE.

The first century BCE is the entire 100 years ending December 31 1 BCE, and starting January 1 100 BCE.

The first millenium BCE then started January 1 1000 BCE and ended December 31 1 BCE.

The fourth millenium BCE started January 1 4000 BCE and ended December 31 3001 BCE.

It is a little odd. The numbers of the years centuries and millenia go in reverse order, like the number line. But the dates within a given year go forward as usual.