I think all the the existing ideas focus on the expiry date. But there are some instances where the food should be consumed 'within 3 days from opening'. The trouble is, when was it opened? Quite often, it's my wife that opens it, but after it has been in the 'fridge for a while, I don't know how long.
So to be safe, I tend to throw it.
It seems most of the items in question are vacuum sealed, or at least, could be.
What I think could work in this case is a plastic vial with a system for allowing the chemical inside to start travelling once the vacuum has been broken. The chemical travels and counts off the days along a scale. At least I would know that ham was opened 1 day ago, or 7 days ago.
The vial could be stuck on the inside of the jar lid as well.