A zero-day (also known as a 0-day) is a vulnerability in software or hardware that is typically unknown to the vendor and for which no patch or other fix is available. The vendor has zero days to prepare a patch as the vulnerability has already been described or exploited.
If it’s a zero day then Microsoft didn’t know about it. If Microsoft knew about the exploit for a year it was not a zero day.
Zero Day just means that you have zero days to fix it before it becomes a problem. Doesn’t mean that you actually take zero days to fix it.
What? No it doesn’t, it means that the exploit has been known for zero days, aka it’s an unknown exploit.
From wiki