Atheists: Why do you use the term "common era" to describe Christ Era,and also to explain the meaning of, C.E.?

When most people who have ever lived on planet earth are alive in 2012?

And, why use the term Before Common Era to describe B.C.E., the period before the birth of Christ?

Is the reason, to deny there was a Christ in born in the 1st. Century?
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