The keyword phrase "September 11th" or "11th September" functions primarily as a proper noun phrase. It acts as a single grammatical unit to name a specific calendar date which, due to historical events, has become the designation for a particular set of attacks and their aftermath. As a proper noun, it identifies a unique entity, similar to names like "World War II" or "the Great Depression."
In syntactic analysis, this proper noun phrase can serve several roles within a sentence. It most commonly functions nominally, acting as the subject ("September 11th changed global politics.") or as the object of a preposition ("The museum is dedicated to the events of September 11th."). However, it can also function as an adjectival modifier, specifically a noun adjunct, to describe another noun ("the September 11th attacks," "the post-September 11th era"). In this adjectival role, the proper noun phrase gives a specific attribute to the noun it modifies, but its fundamental grammatical identity remains that of a noun.
Understanding its primary classification as a proper noun is crucial. This grammatical status reflects its semantic function: it does not merely describe a day but names a singular, momentous historical event. This distinction elevates the phrase from a simple date reference to the title of a specific, globally recognized occurrence, carrying significant connotative weight.
