My understanding of the zero-day definition is that real exploits are in the wild at the point of disclosure, so they *are* being exploited from the start (effectively it is actively being attacked on day zero of the vunerability being made public). Not that patches don't necessarily exist, but the race to patch before an exploit is out there has been lost before it started.
Although, like everything these days the definition has been misused so much it probably doesn't matter