What part of speech is month in the sentence September ninth year?

In the phrase "September ninth year," the word "September" is functioning as a noun.

Here's why:

* It names a specific month: September is the ninth month of the year.

* It acts as the subject of the implied verb "is": You could add "is" to make the phrase a complete sentence: "September is the ninth month of the year."

Even though the phrase isn't a complete sentence, "September" still acts as the subject of the implied verb, indicating that it is functioning as a noun.