Subscribe to The Chicago Manual of Style Online
Usage and Grammar
Q. To me, 12:00 is either noon or midnight, never a.m. or p.m. I keep seeing copy that says “before 12 p.m.” and I can’t convince the copywriters that this is confusing. Can you cite any rule that would clarify this once and for all?
A. Yes. Please see CMOS 9.38: “Except in the twenty-four-hour system (see 9.39), numbers should never be used to express noon or midnight (except, informally, in an expression like twelve o'clock at night). Although noon can be expressed as 12:00 m. (m. = meridies), very few use that form. And the term 12:00 p.m. is ambiguous, if not illogical.”