But chances are you won't know that because in most states expiration dates aren't required, and where dating is mandatory it is inconsistent and confusing.
And that's just for the highly perishable items like poultry and dairy products. You may need a guidebook to decipher the expiration code on a can of beans, but according to the U.S. Department of Agriculture, no such book exists.
Community volunteers working with the University of Southern California found that the problem is particularly acute in lower income, inner city areas, where they found at least one expired product on every third visit to the market.
And on 18 percent of their visits to the store, they found at least three expired poultry, beef and dairy items.
But they also found expired items when they visited stores far beyond their neighborhoods, including other states and the posh suburbs along the beaches to the west of Los Angeles.
"It's an issue that is more widely distributed than I had thought," LaVonna Lewis, a professor in USC's school of policy, planning and development, said in an interview.
For the past decade Lewis has been working with about 90 volunteers from the non-profit Community Health Councils, Inc., in a program designed to get local people involved in monitoring food suppliers in their area.
The volunteers have found that South Central Los Angeles, which is predominately black, is served mostly by smaller mom and pop stores and fast food restaurants in sharp contrast to the wealthier areas to the west.
But five supermarket chains also have a smattering of stores in the poorer part of town, and when the volunteers turned their attention to those stores they were in for a surprise.
Expired foods were found on nearly half the visits to some of those supermarkets, which have not been named because the researchers are still compiling the data for publication in a journal.
But this isn't limited to minority communities in southern California. Inspectors from Westchester County's Department of Consumer Protection found hundreds of expired items when they visited stores in that New York county earlier this year, and some of the items had been expired for more than 31 days. And that, according to health officials there, could be dangerous.
Is it a big deal? Yes, and no.
Lewis said that when her volunteers began keeping track of expired foods they were doing so in the expectation that the foods were unsafe.
"But we've been talking with some food science people and nutrition people and they are telling us it's a quality issue not a safety issue, except for infants and pregnant women and people who may have some kind of auto-immune disease," she said.
That appears to be the consensus among food scientists, although there does not seem to be a lot of research backing it up. The USDA, for example, maintains that an expired product should still be "safe," but it may not taste as good.
"It is not a safety date," according to the agency's food labeling fact sheet.
However, the safety depends chiefly on how the product is handled once it leaves the store. If it's refrigerated, it should be good for a couple of days, according to the agency, but if it smells bad, or if it looks like it has spoiled, toss it, or better yet, return it to the store.
There is no federal requirement to label expired foods, except for infant formula and some baby food, contributing to inconsistencies in the marketplace.
At least 20 states require some dating, but selling expired foods, even at a discount, is rarely prohibited. And different foods are labeled in different ways, ranging from "sell by," to "best if used by," to "use by."
And canned goods, which should be good for two to five years, according to the USDA, frequently carry an encoded expiration date that means nothing to the consumer.
Some expired foods are sold at a discount, which Lewis's researchers call a "manager's special," so at least the buyer knows the quality may be slightly less. But is it safe?
Lewis isn't real sure. "I'm still not 100 percent convinced," she said.
"We don't think people are making informed choices because the information is so hard to find," she added.
She's particularly concerned about residents in poorer neighborhoods who may be especially vulnerable.
"The consequences may be greater in south Los Angeles because of a low health status," she said. "If people are already compromised by chronic disease and other kinds of things, the consequence of eating expired food could be greater."
But this isn't just a problem for the inner city. Consumers Union researchers found 72 items past their "sell by" date in supermarkets in Connecticut, New York, Ohio, Tennessee and Texas. They found no expired foods in markets in Michigan and Washington.
By the way, "sell by" indicates the last day a product should be sold. "Use by" indicates the last day the product is likely to have its full flavor and quality.
Lewis and her colleagues have met with representatives from all five of the supermarkets where expired foods were found. When they laid out their findings, the reactions ranged from "defensive" to "challenging the data," she said. No one from any of the markets seemed shocked, she added.