yougurt stays good way way way past the date on the carton. THink about it, it is a fermented product - The only reason not to feed it would be if there is mold, and then I would just scrape off the mold and discard the immediate layer beneath the mold.
Yougurt is fine for you way way past the date, too - I mean, 6 months past the date and it is still fine, assuming it has been refrigerated.
sell by and use by dates are pretty meaningless. Meat, for example, is often months old by the time it hits your grocery meat case - jars of dressings, mustards, ketchups, pickles, olives, etc- have a couple of good years to them - but sell dates are much shorter. Heck, even coke has expiration dates, and what the heck could happen to water and chemicals beats me.
stepping off the box, here.