It's a stupid term. For the warez scene it makes sense, because a 0 day license exploit has the hallmark of having been available on day 0 for all time.

Using 0 day to refer to "days since disclosure to vendor", well, it becomes a 1-day on the very next day. Which means you can only use the term on one day out of all time.

If we used 0 day to refer to "days since release", as in "iOS 8 was released today and there's already an exploit" then that would be meaningful.