The length of time yogurt is safe to eat after its expiration date depends on whether the date is a “sell by” or “use by” date. Regardless of the date, do not eat yogurt that has a blue-green tint or smells or tastes unusually sour. Most yogurt is stamped with a “sell by” date rather than an expiration date. The yogurt should be good for about two weeks after the “sell by” date, according to Julie Garden-Robinson, a food and nutrition specialist at North Dakota State University.