"It is exactly seven o'clock."
Translation:Kello on tasan seitsemän.
Plus in the previous question we were asked to translate "it's exactly six o'clock" and if we entered kello on tasan kuusi" it was not accepted, it was only accepted without "kello on"
Do you hear it as an answer to a question? Because in that case you can have e.g.
"Mitä kello on?" - "(Se on) tasan seitsemän."
As a statement though, the sentence doesn't really function all that well without "kello". If I've been talking with someone about an wholly unrelated topic and suddenly notice the time, I could say e.g. "Oho, kello on jo seitsemän".