The actual usage is more fluid than that. It wouldn't be a common term if it actually only applied for the first day of every vulnerability being known.
"An attack on a software flaw that occurs before the software's developers have had time to develop a patch for the flaw is often known as a zero-day exploit."
"An attack on a software flaw that occurs before the software's developers have had time to develop a patch for the flaw is often known as a zero-day exploit."
http://www.tomsguide.com/us/zero-day-exploit-definition,news...