- From a logic perspective: The item starts to decay one you open the container where it can be found or you buy it from a merchant. The item did not start to decay while it was in this place before.
Lets find a reason for this: "An archmage or god cursed every usable item in the world to start decaying once a character who is infected with a special tadpole touches it." sounds like one of the more plausible ideas.
I think this can be untangled. Just like real life, things DID decay while at the merchant or in a chest. The number displayed to you would for your immersion be just the rest of its expiration, not the start. If one needs an argument for why I can buy the same health potion at a trader 10 days later with the same expiration date, it would be because they either renew their stock regularly, or because they have a lot better storing options than, you know, lugging it around all day in a backpack full of dangerous stuff
- From a gameplay perspective: Players will find ways to deal with annoying game mechanics. In this case it may be good to sell all consumables one day before they expire. So players are encouraged to visit a shop every time before they rest. This would be very annoying but possible thanks to the fast travel mechanic.
Good point. Of course things might decrease in value as their expiry date goes down so selling one day before would not be a great strategy. The best strategy in that case would be selling after getting an item you don't think you'll need in the near future, which is exactly the point of my suggestion.
