The meeting was moved to 10:30 a.m. tomorrow.
My flight is at 8:10 p.m.
The abbreviations a.m. and p.m. come from the Latin phrases ante meridiem and post meridiem, meaning, respectively, "before noon" and "after noon." As with many other Latin abbreviations that we use in English—e.g., i.e., et al., etc.—the preferred style is to use lowercase letters with periods.
The Chicago Manual of Style, the AP Stylebook, the MLA Style Manual, and Merriam-Webster’s Collegiate Dictionary all recommend this style. But keep in mind that it is a question of style, so some people may disagree. If your company has an in-house preference for small caps with no periods, that’s what you should use.
Related fun fact: It’s not technically correct to say 12:00 p.m. and 12:00 a.m.; noon cannot logically be after noon, and midnight could be either twelve hours before or twelve hours after noon. If you want to be a real stickler, say 12:00 noon or 12:00 midnight.
Of course, we could solve both of these problems by using the twenty-four-hour clock. British people are laughing at us for even needing to consider these questions. But Americans seem about as likely to convert to the twenty-four-hour clock as to the metric system, so we’re stuck with these debates for the foreseeable future.
Do you have a preference for one of the other styles of writing a.m. and p.m.? Do you cringe when other people use 12:00 p.m.? Or do you roll your eyes about that technicality? Do you use a twenty-four-hour clock? Share your thoughts below.