When you say "dated for expiration," is the date a "sell by" date, a "use or freeze by" date, or a "best by" date or maybe one that says "discard after"? There is a HUGE difference between a "Best by" date and an "Expiration" or "Use by" date--or, for that matter, a "Sell by" date. The "discard after" is the only one that I'd get excited about.
"Sell by" date: The last date a store is permitted to sell the product as "fresh." If it's poultry, fish, or other perishable meat, you should be able to keep it under refrigeration for at least 24 hours up to 72 hours or to freeze if it can be frozen (and keep for a normal time in the freezer). When used within the 72 hours or when removed from the freezer, the product should be safe to eat. If it's laced with preservatives and in a vacuum wrap, it should be good for a while after that.
"Best by" date: The product will deteriorate slightly over a long period of time. The product is going to look its best, taste its best, and have the best texture prior to the "Best by" date. After that, the deterioration may be noticeable, although the product is still safe to eat. Peanut butter will have an "off" or rancid taste. Condensed milk will caramelize in the can. Things like bread may show mold. Cookies and crackers may have a soggy texture or taste stale. Baking powder may not rise if kept past its "Best by" date (but it won't make you sick). Noodles and other dry pasta should be fine to use as long as they haven't been subjected to high humidity or invaded by insects, even up to a couple of years after the "Best by" date.
"Expiration" or "Use by" date. This applies to things that do spoil rapidly even under appropriate storage conditions. Eggs, milk, most meat, medications . . . all are likely to have a "Use by" date after which the safety of the item or the potency of medications may be in question. The usual test for milk and meat is the "sniff" test--smell it, and if it smells bad, toss it. Eggs can be sunk in a bowl of water. If the egg sinks, it's okay to cook and eat. If it sinks under the water but stands upright, you wouldn't want to scramble or hard-boil it, but you should be able to use it in baked goods without a problem. If it floats, toss it. Fresh meat may be starting to decompose, and you wouldn't want to eat it. Cured meat, however, is a different story.
If your corned beef were cured the traditional way, it would have a long life, and cooking it would kill any bacteria remaining. However, you might want to read the package *fine print* carefully to see what the package actually contains. And here's the USDA recommendation: http://www.fsis.usda.gov/pdf/corned_beef_and_food_safety.pdf