You can use the following abbreviations with numerals in formal academic writing. Notice that AD precedes the numeral; all other abbreviations follow the numeral. Today, BCE and CE are generally preferred over BC and AD, and periods in all four of these abbreviations are optional.

399 BCE (“before the common era”) or 399 BC (“before Christ”)

49 CE (“common era”) or AD 49 (anno Domini, Latin for “year of our Lord”)

11:15 AM (or a.m.)

9:00 PM (or p.m.)

For abbreviations, you may use full-size capital letters or small caps, a typographical option in word-processing programs.