wintertime vs time of year

wintertime and time of year both are nouns.

Word	Noun	Adjective	Verb	Adverb
wintertime	Yes	No	No	No
time of year	Yes	No	No	No

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

wintertime: the coldest season of the year; in the Northern Hemisphere it extends from the winter solstice to the vernal equinox
time of year: one of the natural periods into which the year is divided by the equinoxes and solstices or atmospheric conditions

Other hypernyms of wintertime include season.

wintertime (noun)	time of year (noun)
the coldest season of the year; in the Northern Hemisphere it extends from the winter solstice to the vernal equinox	one of the natural periods into which the year is divided by the equinoxes and solstices or atmospheric conditions