‘Q. Is there any chance that “am” and “pm” will become acceptable as correct forms of “a.m.” and “p.m.”?
A. There are six ways to write the abbreviations for ante meridiem (before noon) and post meridiem (after noon):
All caps with periods: 10 A.M., 10 P.M.
All caps without periods: 10 AM, 10 PM
Small caps with periods: 10 A.M., 10 P.M.
Small caps without periods: 10 AM, 10 PM
Lowercase with periods: 10 a.m., 10 p.m.
Lowercase without periods: 10 am, 10 pm
Each of these—including “am” and “pm”—is a legitimate choice. For nearly a century, Chicago’s preferred form was the third: small capital letters with periods. This preference, however, applied only to published documents (among other factors, small capitals weren’t an option on typewriters).
When we changed our preference to “a.m.” and “p.m.” (in 2003, with the publication of CMOS 15), the growth of computers in writing and publishing played a role: small caps require extra steps to apply, and they don’t always translate well across applications (when they’re even available). We could have flipped a coin and settled on all-caps “AM” and “PM” (but not “A.M.” and “P.M.”; Chicago style now omits periods in abbreviations that include two or more capital letters). When we instead chose lowercase “a.m.” and “p.m.,” we liked the fact that they’re unambiguous (“AM” and “PM” both have a number of other meanings), and we hoped the periods would help readers recognize in any context that these are abbreviations, not words.
But if you don’t like the periods, don’t fret: Merriam-Webster labels “am” and “pm” as British variants, so you’re hardly alone in your preference. If you’re being published, however, be prepared to defer to your publisher’s house style, whatever that may be.”
Source: FAQ Item