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Q. I am having an argument with our law clerks. I do not believe that a comma is needed when referencing a date range—e.g., “The case was active from November 3, 2021 to November 30, 2022.” My law clerks insist that a comma belongs after 2021 (between the dates), and I say that when a date range is preceded by a preposition, a comma is unnecessary. Can you provide me with a definitive answer? Thank you.
A. Most style guides published in North America (where the “Month Day, Year” format is preferred) will tell you to use that second comma (the one after the year). This includes not only CMOS (see paragraph 6.38) but also the guides from the Modern Language Association (MLA), the Associated Press (AP), the American Psychological Association (APA), the American Medical Association (AMA), and the US Government Publishing Office (GPO). The guides from Microsoft and Apple also support this rule.*
The idea is that the year is parenthetical—November 3 (2021)—and in your example this usage is relatively straightforward. But when the date is used as a modifier before a noun, the result can seem awkward, and some guides—including CMOS (in paragraph 5.83)—recommend rephrasing if possible:
The January 10, 2023, decision was unexpected.
or, less awkwardly,
The decision of January 10, 2023, was unexpected.
As for legal contexts, The Bluebook (the legal citation guide published by the Harvard Law Review Association) doesn’t seem to specify how to punctuate dates outside of citations (where a comma might follow a year but for other reasons). But we’d be surprised if The Bluebook’s editors didn’t support the additional comma in a sentence like yours. (A look at the Harvard Law Review’s website suggests a preference for the second comma.)
Verdict: Your law clerks aren’t wrong in this case.
* MLA Handbook (9th ed., 2021), 2.13; AP Stylebook (56th ed, 2022), under “comma”; Publication Manual of the American Psychological Association (7th ed., 2020), 6.3; AMA Manual of Style (11th ed., 2020), 188.8.131.52; GPO Style Manual (2016 ed.), 8.53; Microsoft Style Guide, “Commas” (June 24, 2022); Apple Style Guide (October 2022 ed.), under “dates.”