The easy way to say when something happens is to use the preposition в and name the hour and the number of minutes after the hour. Russian works just like English here: в вћсемь пятнђдцать "at 8:15". The hard way to tell time is the more usual way. These comments concern 'after' or 'past' the hour, when the minute hand points to the right side of the clock face.
MINUTES: use в + Accusative case of the numeral plus the word for 'minute'.
HALF HOUR: use в + Prepositional (в половќне) or в + пњл-
HOURS: use the Genitive of the ordinal numeral.
Here are some answers to Когдђ? Во скћлько? (very colloquial) or В котћром часџ? (note the Locative case of чђс).
Ї уезжђю в пїть минџт пљрвого. I'm leaving at five after twelve. (Lit. 5 minutes of the 1st)
Ї уезжђю в пїть минџт вторћго. I'm leaving at five after one. (Lit. 5 minutes of the 2nd)
Ї уезжђю в двђдцать пїть минџт трљтьего. I'm leaving at twenty five after two. (Lit. 25 min. of the 3rd)
Ї уезжђю в двљ минџты четвёртого. I'm leaving at two after three. (Lit. 2 minutes of the 4th)
Ї уезжђю в однџ минџту пїтого. I'm leaving at one after four. (Lit. 1 minute of the fifth hour)
Ї уезжђю в чљтверть шестћго. I'm leaving at quarter past five. (Lit. quarter of the 6th hour)
Ї уезжђю в половќне десїтого. I'm leaving at half past nine. (Lit. half of the 10th hour)
Ї уезжђю в пњлдесїтого. (Same meaning, but more colloquial.)