One of my nemeses in writing and editing is the issue of how to write expressions of time.
For example, which is correct: “2:00 pm”, “2:00 p.m.”, “2:00 PM”, or “2:00 P.M.”? Capitals or lowercase? Periods or no periods?
It’s enough to drive an editor batty.
The Chicago Manual of Style, 17th ed. (9.37) and The Publication Manual of the American Psychological Association assert that it should be lowercased with periods (2:00 p.m.), probably because the abbreviations p.m. and a.m. stand for post meridiem and ante meridiem, respectively.
However, Scientific Style and Format (CSE) states that the abbreviations should be in capital letters without periods (2:00 PM).
However you write it, what is most important is to consistently follow the style guide you are using. Please don’t write 2:00 p.m. on one page and 2:00 PM on another, for example. That is my golden rule.
Now then, please also remember that 12:00 a.m. is midnight, and 12:00 pm is noon.
Fellow editors, how do you prefer to write expressions of time?