dog days vs period of time

dog days and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
dog days	Yes	No	No	No
period of time	Yes	No	No	No

As nouns, period of time is a hypernym of dog days; that is, period of time is a word with a broader meaning than dog days:

dog days: the hot period between early July and early September; a period of inactivity
period of time: an amount of time

Other hypernyms of dog days include period, time period.

dog days (noun)	period of time (noun)
the hot period between early July and early September; a period of inactivity	an amount of time