How do you count millenniums in BC?

Answer 1

BC + AD - 1

You have to subtract 1 after adding the year BC to the AD year. There is no year Zero, so the calendar is not a proper timeline. This is not a matter of interpretation (like the new millennium arguments).

If you were born in 1 B.C., then you were one year old in A.D. 1. That's 1 + 1 - 1.

If the start and end years are both BC, then it's a simple subtraction of the smaller number from the larger number, just as you would calculate your own age today.

Note that "BC" is properly written after the year, but "AD" (Anno Domini) goes before the year. If you use the more inclusive "Common Era" notation, then both "CE" and "BCE" go after the year.

Answer 2

Same as AD - 1000 years each.

Since there is no zero year, each millennium goes from x001 to (x+1)000

[that's like the second millennium went from 1001 to 2000.]

It works the same when you're counting backwards -

the only problem is that all BC dates are really just approximations.