forenoon vs period of time

forenoon and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
forenoon	Yes	No	No	No
period of time	Yes	No	No	No

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

Other hypernyms of forenoon include period, time period.

forenoon (noun)	period of time (noun)
the time period between dawn and noon	an amount of time