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A.D., B.C., C.E., B.C.E.

Small caps, periods, no space.

A.D. precedes the year but follows the century.  Use only in early dates when the era might be in doubt.

Though A.D. means "in the year of the Lord," supply the pertinent English preposition; do not add the word "year":
  Pompeii was buried August 24, A.D. 79; around A.D. 79
Augustus died in A.D. 14; not in the year A.D. 14
He died in the first century A.D.
June, A.D. 52
A.D. 300 to 1000
100 B.C. to A.D. 50

B.C. follows the date; use comma with five figures or more:

522 B.C.
2500 B.C.
12,000 B.C.
third-century B.C. sculpture
15th-century B.C. sculpture
8000 to 3000 B.C.
100 B.C. to A.D. 50

An alternative system gaining popularity, especially in academic literature, uses C.E. (meaning the Common Era and used in place of A.D.) and B.C.E. (before the Common Era).

National Geographic generally uses the traditional terms, A.D. and B.C.