CMOS 16 CHANGES, #5
All caps or small caps or lowercase, with periods or without, for AD/BC and AM/PM?
The 16th edition of The Chicago Manual of Style (rule #10.39) says that although AD and BC have traditionally appeared in small caps (with or without periods), the new recommendation is full capitals without periods. (AD precedes the year number; BC follows it.)
Rule #10.42 says that, although AM and PM sometimes appear in small caps (with or without periods), their suggested preference is now the lowercase form, with periods: a.m. and p.m.