MINUTES: без + Genitive of the numeral(s).
HOUR: Nominative/Accusative of the numeral.
The literal meaning of без is 'without', but here it has the meaning 'minus'. Thus, без десятќ вћсемь might be thought of as 'eight o'clock minus ten [minutes]'.
There is no difference between telling the time and saying 'at' what time:
Сейчђс без десятќ вћсемь. It is now ten of eight (7:50).
Ї обІчно встає без десятќ вћсемь. I usually get up at ten of eight (7:50).
If the number of minutes is not a multiple of 5, many speakers add the word for 'minute':
Сейчђс без двадцатќ пятќ вћсемь. It is now twenty five of eight (7:35).
Сейчђс без двџх минџт вћсемь. It is now two minutes of eight (7:58).
Сейчђс без однћй минџты вћсемь. It is now one minute of eight (7:59).