To find out “what time is it?” Russians ask either “Кото́рый час?” or “Ско́лько вре́мени? / Ско́лько сейча́с вре́мени?” To answer that question when the time is on the hour, you just use the word сейча́с plus the times as reflected below. For example, if it is 6 o'clock, you would say Сейча́с шесть часо́в.
Three forms of the word час are used in telling time, and the form of час depends on the quantity of hours mentioned. After the number 1, we use the form час (nominative singular), after the numbers 2, 3 and 4 we use the form часа́ (genitive singular of час), and after 5 (and through 20) we use the form часо́в (genitive plural of час).
This pattern of case usage is standard when counting objects in Russian, and you will meet more examples of this in coming units.
The 24-hour clock is used in Russia for schedules and public announcements, in print and in the media. In the 24-hour clock in Russian, 1pm becomes трина́дцать часо́в and for the hours up to два́дцать часо́в, we continue to use the form часо́в. At 9pm or 21 hours, we say: два́дцать оди́н час (back to the nominative singular of час), and for 22, 23, 24 we say, два́дцать два часа́, два́дцать три часа́, два́дцать четы́ре часа́ (back to the genitive singular of час).
Clock face courtesy of TeeKay.