If you purchase an iPhone from Apple directly or from an actual Authorized Apple Seller, the warranty starts the day the phone was purchased and has nothing to do with the date the phone was activated.
If you purchase an iPhone from a seller is who is NOT Apple or an actual Authorized Seller, the warranty on the phone begins the day the phone left Apple or the Authorized sellers inventory.
So, if you buy a phone from someone other than Apple or an Authorized seller and the phone sat on a shelf for a month or more, the warranty would have started the date it left Apple's inventory. In other words, activation is not a factor for warranty at all. The date it left Apple or an Authorized Sellers inventory is the date warranty begins.
Only buy from Apple or an Apple Authorized Seller and you won't have any issue. Buy from someone not authorized, and you take your chances.