there's a lot of misunderstanding about food "freshness" dating floating about. the Federal laws (i.e. apply everywhere) are very limited -
/q
Is Dating Required by Federal Law?
Except for infant formula and some baby food (see below), product dating is not generally required by Federal regulations.
/uq
ref:
http://www.fsis.usda.gov/Fact_Sheets/Food_Product_Dating/index.asp
the "not generally required" is an interesting catch phrase - (some) shellfish must carry not only harvest date, but place of harvest, for example - mussels, clams . . .
past that it's up to state laws - so there's a whole bunch of those - few of which "agree" - and recently while researching some state laws I found state examples where state laws explicitly do not require manufacturers or retailers to "pull" out-of-date stuff from the shelf, nor legally ban the sale of stuff 'past the date.'
essentially manufacturers establish their own guidelines for sell by or use by dates - typically based on their concepts of 'product / quality' deterioration.
another oddity is for example eggs - it's legal, at least in some states - for egg suppliers to remove 'expired' eggs from the case, rewash/reprocess them and put a 'new' date on the package. our local market recently made a big deal out of "our eggs are never re-processed"
it certainly pays to know your own local / state laws - regrets I've not found a site that summarizes all the known state regulations.