mid-December vs period of time

mid-December and period of time both are nouns.

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

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

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

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