dog days vs time period

dog days and time period both are nouns.

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

As nouns, time period is a hypernym of dog days; that is, time period 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
time period: an amount of time

Other hypernyms of dog days include period, period of time.

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