Morning LatioR,
I have read that the rubber will hardened if exposed to UV, heat and water, usually this apply to used tyres. But for brand new that are properly sealed can last for quite some time but nobody dare to commit the lifespan.
You can still drive with your stock toyo tyre for more than 3 yrs for 47,000km already, so dun tell me if the same manufactured date that is sealed cannot be use ? Logic ? Maybe the tyre shop will sell at a discount for long storage for new tyres ? And I have not heard of expiry date for tyres.
Anyway it a good practice to check your tyres regularly. 
There is actually a greater possibility for new tyres in storage to deteriorate faster as the rubber is not "flexed", ie. not being rotated, as fitted on a car and being driven.
Even your pencil eraser will harden if not used over a prolonged period.
That is also how a new tyre fitted to a car can get flat spot if the owner travels overseas frequently, seldom uses his car and the parked car's tyres are left to carry the weight of the car on just one spot.
Unless the sealed tyres are stored in a temperature and humidity controlled environment, there is a even higher possibility of mid dew and mould and fungus building up on the tyres, beneath the warmth of the wrappers.