The keyword term is a noun phrase. Its primary grammatical function is to act as the subject or object within a sentence, referring to a specific historical event defined by its date and its temporal distance from the present. The core of the phrase is the proper noun "September 11th," which is specified further by the temporal modifier "23 years ago."
This phrase is a composite structure. "September 11th" serves as the head noun, identifying a specific day. The subsequent part, "23 years ago," functions as a post-nominal modifier, an adjectival or adverbial phrase that specifies which instance of that date is being referenced. "23 years" is a measure phrase, and the adverb "ago" anchors it to a point in the past relative to the moment of communication. While it can function adverbially to answer the question "When did an event occur?", its capacity to stand alone as the subject of a sentence (e.g., "[The phrase] was a turning point...") confirms its classification as a noun phrase.
Identifying the term as a noun phrase is crucial for an article because it establishes the event itself as the central topic of discussion, rather than just a time marker. This allows the article to treat the historical moment as a concrete subject that can be analyzed, debated, and possess attributes (e.g., a legacy, consequences, or significance). This grammatical classification supports a thematic focus, enabling the construction of sentences where the event is the primary agent or a concept to be examined in depth.
