early days vs period of time

early days and period of time both are nouns.

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

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

Other hypernyms of early days include period, time period.

early days (noun)	period of time (noun)
an early period of development	an amount of time