1. Use figures for decades: the 1990s, the ’90s, the mid-1990s.

2. Use an apostrophe before the decade if abbreviated as in the ’80s, the turbulent ’60s.

3. Spell out the decade when used at the beginning of a sentence: Nineteen-eighties business models are no longer sufficient.

4. Follow the “less than 10” as in the fifth century, the 15th century, mid-eighth century, mid-12th century, ninth-century literature, 17th-century poets.

5. B.C. follows the year or century as in 88 B.C. or fifth century B.C.

6. A.D. comes before the year as in A.D. 96 but after the century as in second century A.D.

Note: A.D. is not always necessary if the century is obvious in the text as in this example:

In the fifth century, Rome fell.

7. Do not use C.E. and C.E. for B.C. and A.D., respectively.

8. C.E. (before the Common Era) and C.E. (the Common Era) are mostly used in academic writing.

For Dragoman copy, use B.C. and A.D.

 

Source
Dragoman Style Guide for Figures