"What time is it?" translates to "Która (jest) godzina?", so "Which hour (is it)?".
Then if you really wanted to say "I do not have the time" rather than just "I'm sorry, I don't know", that would be "Nie wiem, która (jest) godzina".
"Nie wiem czasu" definitely isn't correct, because the verb "wiedzieć" is not used with direct objects. That would need to be "Nie znam czasu". But that wouldn't really be said anyway.