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Q. A quandary: I’m seeing September 11th (added “th”) in the New Yorker magazine, where editing is usually superb, but somewhat antiquated. The New York Times refers to the date as Sept. 11 or 9/11. Please give me a rundown of your recommendations for this particular date, including use as an adjective (September 11 tragedy?). Or is it still too soon to have a set standard? Thanks. I’m probably the 911th person to ask you this.
A. In written text, Chicago’s rule is to write a cardinal rather than an ordinal, even though the number may be pronounced as an ordinal:
the events of September 11
the September 11 tragedy
September 11, 2001
When a day alone is mentioned, it is usually in the form of an ordinal but spelled out:
September 10, 2001, was the last day of its kind in the United States. The tenth will therefore always be important, even as it stands in the shadow cast by the eleventh.
As for an abbreviated form for September 11, “9/11” works fine (but see CMOS 9.35).