Do you put periods in AM and PM?

While organizing a guide on the use of the period, I came upon this question of whether you put periods in the abbreviations AM and PM.

According to most major style guides, like The Chicago Manual of Style and the AP (Associated Press) Stylebook, a.m. and p.m. should be written in lowercase and separated by periods when used in sentences.

As a reminder, these two abbreviations stand for the Latin terms “ante meridiem” and “post meridiem,” literally meaning “before noon” and “after noon.”

The grammarist.com reports that the term “p.m.” was in use as early as the 1640s, but the term “a.m.” was not in use until the 1760s, even though timekeeping in twelve-hour increments became standard in Europe the century before. Most people follow twelve-hour clocks, with scientific and military groups mainly using twenty-four hour clocks.

Using a.m. and p.m. with numerals emphasizes exact time, generally used in official and academic documents. Informally, often words spell out the time.

Formal: The committee is meeting at 2:00 p.m. on Thursday.

Informal: On Thursday, I’ll be golfing at two in the afternoon.

When spelling out time zones, separate from the abbreviation with a comma:

The webinar will run from 11:30 a.m. to 1:00 p.m., central standard time.

Though the abbreviations a.m. and p.m. are generally presented in lowercase when used in running text, some guides say that they may also be capitalized, especially when used in headings, signs, and notices.

For example, for the 4.23.22 Recycling Day at the Honest Weight Coop in Albany, NY, the sign read:

SATURDAY, APRIL 23rd  * 11AM – 3 PM.

Guides differ in the use of periods with the capitalized abbreviations, some say they are omitted, others say their use is permitted. (In British usage, periods are often omitted even with the lowercase abbreviations.) The examples below illustrate both format options:

We will meet for dinner Thursday at 7:00 PM.

We will meet dinner Thursday at 7:00 P.M.

When capitalized, the abbreviations are often put in small capitals, which makes them easier to read. 

Style guides also recommend separating the time numeral from the abbreviation with a space, as shown in all the examples above.

FYI: The words noon and midnight are written in lowercase, but writing “12 noon” or “12 midnight” is not standard usage because it is redundant.

(Sources: editorsmaual.com; writingexplained.org; grammarist.com; capitalizemytitle.com)

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