A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Eras
Abbreviations
Part III. Style 20 spelling

There are various systems for designating eras, all of which use abbreviations with numerical dates. The most common system uses the terms BC and AD, but some disciplines use different designations, such as those listed below. Use the relevant abbreviations throughout your paper; put them in all capitals, without periods. AD precedes the year number; the other designations follow it (see also 23.2.4 and 23.3.2).

BC - before Christ

and

AD - anno Domini (in the year of the Lord)

BCE - before the common era

and

CE - common era

BP - before the present

MYA (or mya) - million years ago