Is unopened yogurt still safe to eat after the “sell by” or “best before” date?

Yogurt is still safe to eat after the "sell by" or "best before" dates, within limits.   Dannon claims their products, if kept sealed and refrigerated, can be used 2-3 days after the "best before" date imprinted on their containers.  Stonyfield Farms  (a manufacturer of organic yogurt)  explains their "sell by" date saying, "Sometimes the products can still be used up to 7-10 days beyond this [“sell by”] date. After that, we don't recommend usage."  A consumer relations representative with the company went on to explain that beyond this period, the bacterial cultures, which act as a preservative,  start to die off.  If fruit products are added to the yogurt, fermentation begins.  
 
Sources: 
Dannon recorded consumer help line:  1-877-DANNONUS
Interview with Dannon consumer relations representative.  (name not recorded)
Stonyfield Farms recorded consumer help line:   1-800-PRO-COWS (776-2697)
Interview with Stonyfield Farms consumer relations representative. (name not recorded)