wartime vs period of time

wartime and period of time both are nouns.

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

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

Other hypernyms of wartime include period, time period.

wartime (noun)	period of time (noun)
a period of time during which there is armed conflict	an amount of time