When does iPhone warranty start.

I’ve bought a new sealed iPhone 16 pro from eBay, when I received the phone I contacted Apple support with the serial number to find out what warranty was left on it, they told me the phone is out of warranty and was likely purchased over a year ago. They also said that I would need to activate the iPhone to confirm this though. I’ve not done this yet because if I open the phone I can’t return it to the seller if need be.

my question is when does the warranty start? When I search this up I get mixed answers some say on activation date and other say on purchase date.

thanks.