grace period vs period of time

grace period and period of time both are nouns.

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

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

grace period: a period of time past the deadline for fulfilling an obligation during which a penalty that would be imposed for being late is waived, especially an extended period granted as a special favor
period of time: an amount of time

Other hypernyms of grace period include period, time period.

grace period (noun)	period of time (noun)
a period of time past the deadline for fulfilling an obligation during which a penalty that would be imposed for being late is waived, especially an extended period granted as a special favor	an amount of time