AM and PM / a.m. and p.m. 

However, as likely expected, at least one style manual had to have a completely different take on the matter. In this case, we actually have two oddities. First is Microsoft (2012), which suggests using the twenty-four-hour notation generally (more on that later), but then insists on using A.M. and P.M.—yes with uppercase and punctuation—when using an initialism. The second anomaly comes from The Economist (2018), which recommends using am and pm—lowercase and no punctuation. (To be fair, while Oxford does not follow the no-punctuation rule, it does note that “some styles omit the punctuation.”) Garner gives a great reason for avoiding the lowercase with no punctuation, “am (as opposed to a.m.) looks like the verb.” Can we really argue with that?

General recommendation: If not otherwise indicated in your style manual, use lowercase with punctuation, a.m. and p.m., as it is the most widely accepted form. Shy away from the Microsoft and The Economist anomalies—these will generally be considered incorrect.

Open or Closed Space

The issue of whether to add a space between the numeral and the initialism actually came up recently in my work. I scoffed at the idea of using a closed form. Who would do such an atrocity? However, I should have done my due diligence to see other perspectives before giving a recommendation. After reviewing several style manuals, I found one, yes just one of thirty, that recommends using the closed form. The Economist says “lower-case abbreviations should follow immediately, with no space.”

Since The Economist recommendation seems quite rare in the world of style guides, I would recommend avoiding the closed form unless explicitly asked to do so. Most editors will consider it an error. Exceptions could probably be made for an advertisement, placard, or poster, but for the most part, add the space.

Using Numerals and Time Spans

Writing time with numerals is common knowledge enough; add a colon ( : ) between the hour and minute values, and then add the appropriate initialism, a.m. or p.m. So, based on that, the following are generally accepted: 10:25 p.m., 5:30 a.m., 12:00 p.m. (noon), and 12:00 a.m. (midnight). Easy-peasy, right? Of course not, you would be remiss to think that it would be as easy as that, nothing ever is with English usage and style.

Colon usage between the hour and minute values is common enough, but another, arguably even more common, approach outside of North America is to add a decimal point ( . ) between the hour and minute, as in 3.40 a.m. or 4.20 p.m. (Oxford, The Economist).

It is also usual enough to omit the double zeros when writing whole hours, for example, 7 p.m., 10 a.m., and 11 a.m. That looks easy enough; however, there must be some special considerations for mixing whole and fractional hours, no?—curiously enough, no. Combining both 7 p.m. and 7:30 p.m. in your work will not raise eyebrows. (But if writing times in a table, it is good idea to add the zeros for better alignment.) Even with time spans, it is okay to have a combination, as in “7 p.m. to 9:30 p.m.” Another thing to note about time spans is that you can use a dash (not a hyphen): 7 p.m.–9:30 p.m. or 7–9:30 p.m. Sabin, however, recommends avoiding the dash. (A quick note: the dash used in a span is called an en dash; it is longer than a hyphen but shorter than an em dash. To make an en dash in MS Word, press Ctrl + Num - . That is to say, press the Ctrl key and press - on the numeric keypad.)

Twenty-Four Hours in a Day

One universal in all style manuals that cover the topic is that if a writer is using the twenty-four-hour clock to show time, then no initialism should be used because it creates redundancy—02:00 a.m. is odd since it is impossible to have 02:00 p.m., so be confident in knowing that 02:00 and 14:00 can be used easily enough to tell the time. Oh, and 24:00 and 00:00 are both equally valid to mark midnight; just keep it consistent.

Other Times to Avoid Initialisms

When using expressions like in the morning, in the afternoon, in the evening, and at night, avoid both a.m. and p.m. Adding them creates a redundancy since the expressions themselves tell the reader the time of day. So, it is okay to use 8 a.m. but not 8 a.m. in the morning. Related to this, avoid initialisms with o’clock. Something like 8 a.m. o’clock is poor form. Finally, it is also important to note that if using spelled-out numbers instead of numerals, avoid constructions like eight a.m., instead opt for eight o’clock, eight in the morning, or eight o'clock in the morning.

References

The Associated Press Stylebook 2020–2022. 55th ed. 2020. New York: Basic Books.

The Chicago Manual of Style. 17th ed. 2017. Chicago: University of Chicago Press.

Christiansen, Stacey, Cheryl Iverson, Annette Flanagin, et al. 2020. AMA Manual of Style: A Guide for Authors and Editors. 11th ed. New York: Oxford University Press; 2020.

The Economist Style Guide. 12th ed. 2018. London: Profile Books Ltd.

Garner, Bryan A. 2003. Garner’s Modern American Usage. New York: Oxford University Press.

Microsoft Manual of Style: Your Everyday Guide to Usage, Terminology, and Style for Professional Technical Communications. 4th ed. 2012. Washington: Microsoft Press.

New Hart’s Rules: The Oxford Style Guide. 2nd ed. 2014. New York: Oxford University Press.

Sabin, William A. 2010. The Gregg Reference Manual: A Manual of Style, Grammar, Usage, and Formatting—Tribute Edition. 11th ed. New York: McGraw-Hill.