Sep 11 vs Sept. 11

Sep 11 and Sept. 11 both are nouns.

Word	Noun	Adjective	Verb	Adverb
Sep 11	Yes	No	No	No
Sept. 11	Yes	No	No	No

As nouns, Sep 11 and Sept. 11 are synonyms defined as:

Sep 11 and Sept. 11: the day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs

Other synonyms of Sep 11 include 9-11, 9/11, September 11, September 11 attacks.

Sep 11 (noun)	Sept. 11 (noun)
the day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs	the day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs