mid-August vs period of time

mid-August and period of time both are nouns.

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

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

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

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