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Over the past few weeks, I have been receiving several search queries about the “correct use of am and pm” or similar variants. Although I already know about standard time conventions, I have always wondered about whether or not there are any established grammatical rules for writing the 12-hour clock designations. After spending at least three hours researching this earlier today, it appears that there is no single hard-and-fast rule for expressing a.m. or p.m. when referring to time. The majority of the grammar-based sites that I encountered listed several variants. Some used capital letters or periods while others did not, but most of these sources agreed that all of these variants were acceptable as long as your usage remains consistent throughout your writing.

Eventually I ran across the Associated Press Stylebook guidelines, which state that for journalistic copy we should use the lowercase letter with period format and write the abbreviations as a.m. and p.m. without spaces between the letters or periods. When expressing an actual time, there is one space between the numerical value and the abbreviation, so for example three o’clock in the afternoon would be written as 3 p.m. Since we bloggers (at least those of us who are serious about our work) often like to think of ourselves as journalists, it’s probably a good idea for me to stick the AP standards on this when writing articles. However, for informal writing it doesn’t seem to make much of a difference.

Meanwhile, there were two other questions that came up frequently in searches regarding proper time expression. Here are the answers to these:

What do a.m. and p.m. stand for?

The abbreviation a.m. is Latin for ante meridiem, which means “before midday” (noon); the one for p.m. is post meridiem and means “after midday.” An interesting historical anecdote to this is that the ancient Romans used the ante meridiem expression literally in the sense that noon (as opposed to midnight, which is common in modern usage) was their reference point for telling time. So a Roman time of 3 a.m. literally meant three hours before noon, or 9 a.m. in modern time.

What about noon and midnight? Are these a.m. or p.m.?

Technically, noon cannot be labeled as a.m. or p.m. because it IS the meridian (midday) and cannot by definition be before or after it. Midnight is somewhat trickier because it can be seen as being 12 hours before or after the meridian, depending on one’s perspective. Most sources, however, state that neither expression can be reliably assigned to this exact moment; it is simply the point at which one day passes into the next according to our standard time conventions.

On digital clocks it is common to see noon displayed as 12:00 PM and midnight as 12:00 AM. Technically this is correct because by the time you see the display, it is at least a fraction of a second past noon (or midnight), meaning that the AM or PM designation would now apply.

Of course, if you wanted to avoid the whole issue of what to do with a.m. and p.m. altogether, you could simply express time in the 24-hour format that has become commonplace in many areas of the world. In the United States, however, most of us still use the old 12-hour clock system with the exception of the military and most computer applications.