Dana Gunders says that "use-by" dates are not a good guideline for knowing when something is ready to toss.
A quick look at USDA's food labeling site confirms that the only product for which "use-by" dates are federally regulated is infant formula. Beyond that, some states regulate dates for some products, but generally "use-by" and "best-by" dates are manufacturer suggestions for peak quality.
Suggestions. For peak quality. That's all.
SFoodie recently overheard some people in a grocery store talking about how they'd had to toss out their mustard because the date had expired. A condiment primarily containing vinegar and mustard seeds that was created several millennia ago to be stored in a non-refrigerated environment? Hardly dangerous. One UK organization estimates that 20 percent of consumer food waste can be attributed to customers believing they had to toss food at its "sell-by" date.
Gunders' advice: "If that milk smells rotten, by all means throw it away." That's the approach we take. But in an era when most food is so highly processed that we don't recognize most of the ingredients, it's hard to know when all of them will go rotten.