mid-September vs time period

mid-September and time period both are nouns.

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

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

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

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