"It is late. It is already seven o'clock."
Translation:On myöhä. Kello on jo seitsemän.
It depends on the context. If "kello" is mentioned in another sentence, like a question ("Paljonko kello on?") you don't have to repeat "kello" but instead can just say "se". If the sentence must stand on its own, i.e. there is no other mention of "kello", you have to say "Kello on...".