This generally means, six in the morning, it can be in the evening in context, but we use 24h clock here.
Yeah but no one here says "Kello on jo kahdeksantoista." It is often written as 18, but in speech we always say "kuusi", unless it's some specific time like an appointment at e.g. 18.15 which we can say "Kello kahdeksantoista viisitoista." to make sure does get marked correct for both parties.