I saw an article on food safety a few months ago. The expert said that the way they decide the "sell by" and "use by" dates is by having a panel of people with a heightened sense of taste taste it from one month on up to a few years. Once the taste starts to diminish (often times, normal people without a heightened sense of taste and smell won't actually notice), that is the date they set for. The expert also said that they have never seen someone sick from eating past the sell by date (the only thing that causes sickness is germs, salmonella, etc).
Essentially, the dates have to do with taste and have nothing to do with safety. In the article, a can of 100 year old peaches and oysters were tested and proved to be fine and still contained nutrients.
It was pretty entertaining. Listen to it: