BamaVol and others may remember this recent thread on milk's expiration:
http://www.flyertalk.com/forum/showthread.php?t=592942

As noted in my post in that thread, I have found the date stamped on milk to be inconsistent between brands. In some cases it indicated a week before the milk went bad and in others it indicated that the milk likely would start turning the next day. So it may indeed be a "sell by" date, but the processors may have different notions as to how much time there should be between this date and when the milk is expected to turn.

Post #17 in that thread also describes a test for eggs.

Like others who have commented, I am also suspicious that dates stamped on some medical and food products are more based upon the manufacturers' wanting new sales than the product actually going bad.