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.
“Sell By” Date
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.
“Use By” Date
A traditional expiration date is a date that follows a statement to "use by" that date. Do not eat yogurt past the "use by" date because it could be unsafe to eat.
After a container of yogurt has been opened, it should be tightly covered and eaten within 10 days. If the "use by" date or two weeks after the "sell by" date occurs sooner, then the yogurt is only good until that date.