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.
Click page 2 below. Practice in Leading Newspapers
Here's the lead story in the Nov. 28, 2002, edition of the Washington Post:
Former secretary of state Henry A. Kissinger and former Senate majority leader George J. Mitchell were appointed yesterday to lead a high-profile commission probing the intelligence and security flaws that allowed the Sept. 11, 2001, terrorist attacks to succeed.
It all began with a tip: In the aftermath of the July 7, 2005, suicide bombings on London's transit system, British authorities received a call from a worried member of the Muslim community ….
And it is particularly clumsy when the day as well as the month is given—e.g.: "The court reconsidered the July 12, 1994 privilege order." Stylists who use this phrasing typically omit the comma after the year—and rightly so; in the midst of an adjective phrase (i.e., the date), it impedes the flow of the writing too much. Garner Legal, p. 247.
Click page 3 below.
Here's an example of both situations: (1) using a date as an adjective and (2) using a city-state as an adjective. From the front page of the Washington Post on Mar. 5, 2003:
One of the men captured in the raid last weekend in Pakistan that netted al Qaeda operations chief Khalid Sheik Mohammed allegedly served as paymaster to the Sept. 11, 2001, terrorists and has been named in several other federal investigations . . . .
Later in the same article:
And Hawsawi is named in a false-statements case against Ali S. Marri, a Qatari man who the FBI contends gathered information in his Peoria, Ill., apartment about dangerous chemicals and U.S. infrastructure targets.
Note: In the above passage, notice how the writer began the sentence with "And." Also notice the correct hyphenation of the phrasal adjective in "false-statements case."
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