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9/11, September 11

Both terms, especially 9/11, have become monikers for the events themselves and as such are no longer merely dates.

When using September 11, add the year if the meaning is not clear in context:
            Since September 11, 2001, he has been unable to find work.
            September 11 heroism was the subject of his report.
            We have all been changed by 9/11.