mean solar time vs unit of time

mean solar time and unit of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
mean solar time	Yes	No	No	No
unit of time	Yes	No	No	No

As nouns, unit of time is a hypernym of mean solar time; that is, unit of time is a word with a broader meaning than mean solar time:

mean solar time: (astronomy) time based on the motion of the mean sun (an imaginary sun moving uniformly along the celestial equator)
unit of time: a unit for measuring time periods

Other hypernyms of mean solar time include time unit.

mean solar time (noun)	unit of time (noun)
(astronomy) time based on the motion of the mean sun (an imaginary sun moving uniformly along the celestial equator)	a unit for measuring time periods