Always keep in mind that expiration dating is 98% voluntary. Only a couple...
...of products are mandated by the government to have expiration dates affixed. One is baby foods and formulas, and I can never remember the second one.
It's funny you asked about the boxes. I just recently had to call Hormel since I had an offer on my desk for Stagg Chili in the boxes. Hormel says the cans are good for 3 years, and the boxes are good for 2. But, they said that as long as either container hasn't been breached in any way, the product is "good indefinitely". The dates are on there to indicate best taste and, more importantly, to indicate that during the time period covered by the dating, the full nutritional content on the label is guaranteed to be in the can or box. After that date, with slow degradation of the nutrients, the product will not contain the full amount of nutrients listed on the label. It is still fully viable, just not as high in nutritional quality.
Tuna typically is good for sale for 4 years in the can, and 3 years in the foil pack. Again, that assumes it has been stored properly and the container not breached in any way. And, the same is true of tuna, in that it is good indefinitely, except for a slow degradation of nutritional content and flavor elements.
I would also add that this would be true of most name brand, American-made products. International standards vary widely, if you know what I mean.
Michael