Comma After the Year
When you indicate month, day, and year, put a comma after the day and after the year (unless some other punctuation mark, like a period or question mark, follows the year). Include these commas even if the month-day-year expression serves as an adjective:
On July 1, 2006, the committee dismissed the employee.
We already responded to your July 1, 2006, letter.
A Note on Inevitable Disagreement
Many writers express their displeasure at putting a comma after the year when the expression serves as an adjective, because “it looks funny.”
But this seems to be the rule, and it does make sense. The year is serving in apposition to the month and day, and thus requires commas before and after it. You can design around the problem by inserting a prepositional phrase: Use “letter of January 17, 2007,” instead of “January 17, 2007, letter.”
What’s the Rule?
Above, I said it “seems to be the rule.” Actually, I cannot find any authority on this point at all. The Random House Unabridged Dictionary does say, “A date given in the order of month, day, and year is also followed by a comma.” Random House, p. 2461. But it does not address the issue of using the expression as an adjective. Indeed, why should the punctuation rule change as the expression serves various roles in the sentence?
In the “for what it’s worth department,” I have noticed that leading writers in leading newspapers put in the trailing comma even when the expression serves as an adjective.
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