mid-December vs time period

mid-December and time period both are nouns.

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

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

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

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