To say what time it is, use the Nominative, just as you say what day or what date it is:
Сейчђс чђс.	It is now one o'clock.
Сегћдня средђ.	Today is Wednesday.
The word сейчђс 'now' is optional; in fast speech it is pronounced as though spelled щђс.
To ask somebody what time it is, you can use the expression in the above conversation (Скћлько врљмени?) or if you want to be more formal you can say Котћрый чђс? The latter expression is the preferred usage of older people.
To say at a particular hour, use в + Accusative; since the word чђс (meaning both 'hour' and 'o'clock') is inanimate, its Accusative form is the same as the Nominative and so are the numerals that go with it.
Скћлько сейчђс врљмени?	What time is it now?
Сейчђс двђ часђ.	It's two o'clock.
Москћвское врљмя — пятнђдцать часћв.	It is fifteen hours, Moscow time.
— Когдђ тІ уезжђешь?	"When are you leaving?"
— В четІре часђ.	"At four o'clock."