I want to target visitors from USA{whole country} from 2pm-6pm. But USA has not one timezone so what would be the best way to do this?
Or only solution is to create campaigns for each time zone?

Good question. It seems indeed like the only solution would be to make a campaign for each time zone but then you'd still have the problem of not being able to target a US timezone.

I'm actually surprised google never implanted this. It's not to hard to divide the US in West, East, Central at the very least