mid-September vs period of time

mid-September and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
mid-September	Yes	No	No	No
period of time	Yes	No	No	No

As nouns, period of time is a hypernym of mid-September; that is, period of time is a word with a broader meaning than mid-September:

Other hypernyms of mid-September include period, time period.

mid-September (noun)	period of time (noun)
the middle part of September	an amount of time