This post first appeared on LinkedIn in 2022 and has been lightly edited for this site.
You are probably wondering what kind of knowledge could possibly be gleaned from such a pedestrian and “easy” topic as time—we all know this boring stuff from primary school. However, bear with me a moment as several nuances emerge when we look at things more closely. For example, did you know that AM (with capital letters and no punctuation) also stands for anno mundi, meaning “in the year of the world” (Chicago 2017, 588; New Hart’s Rules 2014, 200)? Let’s take a closer look at a few other things.
Quick Definitions
The initialism a.m. comes from the Latin phrase ante meridiem, meaning “before midday” or “morning.” The opposite initialism is p.m., coming from the phrase post meridiem, meaning “after midday” or “afternoon” (Chicago 2017, 589; Garner 2003, 39). You may not know, but there is a third form, m. for meridies, meaning “midday” or “noon.” In writing it appears as 12:00 m., but it is hardly ever used today (Chicago 2017, 559, 589). To better understand initialisms in general, see my post on shortened word forms. Well, with that out of the way, let’s get into it.
Uppercase or Lowercase? That Is the Question
The topic of whether to use uppercase (AM or PM) or lowercase (a.m. or p.m.) is not often debated. Associated Press (2020, 17, 232, 297), Chicago (2017, 589), New Hart’s Rules (2014, 195), Sabin (2011, 156, 185), Apple (2009, 171), Hacker and Sommers (2021, 82), the American Psychological Association (2020, sec. 6.32), The New York Times (Siegal and Connolly 2015, s.v. “time”), and several others recommend using lowercase with the punctuation. However, Garner (2003, 39) tells us that while lowercase letters with punctuation is standard, many editors like the look of small caps without punctuation (see also Bringhurst 2004, 48, 88; Felici 2012, 200, 260; Strizver 2014, 198–99). Christiansen et al. (2020, 963, 1121) also differ in their recommendation, calling for AM and PM set in small caps. Small caps take the form of uppercase letters but are the size of lowercase letters (Sullivan and Eggleston 2006, 222).
Example of small caps: The meeting starts at 2:30 pm, but some members will be unable to attend.
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, 235), which suggests using the twenty-four-hour notation generally (more on that later; UNESCO [2004, 32] and ICHCAP [2021, 8] recommend it too), 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, s.vv. “abbreviations,” “time”), which recommends using am and pm—lowercase and no punctuation. (To be fair, while New Hart’s Rules does not follow the no-punctuation rule, it does note that “some styles omit the punctuation” [2014, 195]; Burchfield [2000, 317] accepts either form as long as it is used consistently, and Petelin [2022, 132] notes that the periods are falling out of use.) Garner (2003, 2) 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 (and your house style should settle the question; see Van Laan 2022, 232), 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 two dozen, that recommends using the closed form. The Economist (2018, s.v. “abbreviations”) 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. and 5:30 a.m. For noon and midnight, though, most guides prefer the words themselves, since 12:00 p.m. and 12:00 a.m. are ambiguous (Chicago 2017, 559; Garner 2003, 39; Microsoft 2012, 235; Sabin 2011, 157; Siegal and Connolly 2015, s.v. “time”; Walsh 2000, 174). Easy-peasy, right? Of course not. You would be mistaken 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. (New Hart’s Rules 2014, 195; The Economist 2018, s.v. “time”; Sabin 2011, 156; UNESCO 2004, 32; Guthrie 2021, 117).
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. (Sabin 2011, 156). 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 a 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. (Sullivan and Eggleston 2006, 65; Bringhurst 2004, 80). Sabin (2011, 156), however, recommends avoiding the dash after between or from, as do Chicago (2017, 396) and Garner (2016, 374); Apple (2009, 171) prefers to in running text. (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 (New Hart’s Rules 2014, 195; Sabin 2011, 158; Felici 2012, 260). Oh, and 24:00 and 00:00 can both mark midnight, but 24:00 generally means the end of a day and 00:00 the start of the next (Chicago 2017, 559; Sabin 2011, 158); Microsoft (2012, 235) allows only 00:00.
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 (Associated Press 2020, 17; Garner 2003, 39; Sabin 2011, 156). 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 (Chicago 2017, 558; New Hart’s Rules 2014, 195).
References
American Psychological Association. 2020. Publication Manual of the American Psychological Association. 7th ed. Washington, DC: American Psychological Association.
Apple. 2009. Apple Publications Style Guide. Cupertino, CA: Apple Inc.
Associated Press. 2020. The Associated Press Stylebook 2020–2022. 55th ed. New York: Basic Books.
Bringhurst, Robert. 2004. The Elements of Typographic Style. Version 3.0. Point Roberts, WA: Hartley & Marks.
Burchfield, R. W. 2000. The New Fowler’s Modern English Usage. Rev. 3rd ed. New York: Oxford University Press.
The Chicago Manual of Style. 2017. 17th ed. Chicago: University of Chicago Press.
Christiansen, Stacy L., Cheryl Iverson, Annette Flanagin, Edward H. Livingston, Lauren Fischer, Connie Manno, Brenda Gregoline, Tracy Frey, Phil B. Fontanarosa, and Roxanne K. Young. 2020. AMA Manual of Style: A Guide for Authors and Editors. 11th ed. New York: Oxford University Press.
The Economist Style Guide. 2018. 12th ed. London: Profile Books Ltd.
Felici, James. 2012. The Complete Manual of Typography: A Guide to Setting Perfect Type. 2nd ed. Berkeley, CA: Peachpit Press.
Garner, Bryan A. 2003. Garner’s Modern American Usage. 2nd ed. New York: Oxford University Press.
Garner, Bryan A. 2016. The Chicago Guide to Grammar, Usage, and Punctuation. Chicago: University of Chicago Press.
Guthrie, Neil. 2021. Guthrie’s Guide to Better Legal Writing. 2nd ed. Toronto: Irwin Law.
Hacker, Diana, and Nancy Sommers. 2021. A Pocket Style Manual. 9th ed. Boston: Bedford/St. Martin’s.
ICHCAP (International Information and Networking Centre for Intangible Cultural Heritage in the Asia-Pacific Region). 2021. ICHCAP Style Manual. Jeonju, Korea: ICHCAP.
Microsoft Corporation. 2012. Microsoft Manual of Style: Your Everyday Guide to Usage, Terminology, and Style for Professional Technical Communications. 4th ed. Redmond, WA: Microsoft Press.
New Hart’s Rules: The Oxford Style Guide. 2014. 2nd ed. New York: Oxford University Press.
Petelin, Roslyn. 2022. How Writing Works: A Field Guide to Effective Writing. 2nd ed. New York: Routledge.
Sabin, William A. 2011. The Gregg Reference Manual: A Manual of Style, Grammar, Usage, and Formatting—Tribute Edition. 11th ed. New York: McGraw-Hill.
Siegal, Allan M., and William G. Connolly. 2015. The New York Times Manual of Style and Usage. 5th ed. New York: Three Rivers Press.
Strizver, Ilene. 2014. Type Rules: The Designer’s Guide to Professional Typography. 4th ed. Hoboken, NJ: John Wiley & Sons.
Sullivan, K. D., and Merilee Eggleston. 2006. The McGraw-Hill Desk Reference for Editors, Writers, and Proofreaders. New York: McGraw-Hill.
UNESCO. 2004. Style Manual for the Presentation of English-Language Texts Intended for Publication by UNESCO. 2nd ed. Paris: UNESCO.
Van Laan, Krista. 2022. The Insider’s Guide to Technical Writing. 2nd ed. Laguna Hills, CA: XML Press.
Walsh, Bill. 2000. Lapsing into a Comma: A Curmudgeon’s Guide to the Many Things That Can Go Wrong in Print—and How to Avoid Them. New York: McGraw-Hill.