time of departure vs departure time

time of departure and departure time both are nouns.

Word	Noun	Adjective	Verb	Adverb
time of departure	Yes	No	No	No
departure time	Yes	No	No	No

As nouns, time of departure and departure time are synonyms defined as:

time of departure and departure time: the time at which a public conveyance is scheduled to depart from a given point of origin

time of departure (noun)	departure time (noun)
the time at which a public conveyance is scheduled to depart from a given point of origin	the time at which a public conveyance is scheduled to depart from a given point of origin