You're at the dairy case stretching for the peach yogurt in the back row. It doesn't matter that three containers somersault onto the shelves below, decapitating a strawberry Yoplait and smashing a couple of eggs. As long as you get the peach yogurt with the latest "sell-by" date!

But think about it. What does the "sell-by" date really mean? And, if you buy it by that date, how much longer will it keep?

For that matter, what do all the other dates on product labels mean? That pound of butter in the dairy case has a "best-if-used-by" date on it. That doesn't tell the supermarket the last day it can sell it, does it? And will anything happen to you if you eat it beyond that date?

Both the yogurt and butter carry examples of "open dating," a food-product dating system that took off during the 1970s in response to growing consumer interest and as an aid to supermarkets in product rotation and quality assurance. Since then manufacturers of national brands as well as retailers of house-label brands have been expanding the number of products on which open dates appear. In fact, some states and localities -- the District, Maryland and Virginia included -- have various laws requiring open dating on perishable products although there are no federal regulations requiring open dates on foods.

Yet there is no uniformity, no single system to provide consumers with consistent information either about how long the food can be sold by the supermarket or how long it can be expected to remain fresh.

Obviously, some products do not have dates. On some that are marked, it's difficult nevertheless to find the date. And on food packages that carry dates, many don't indicate what the date means -- there's just a date. In short, it's confusing.

Here is, at least, a framework to start from -- the basic methods of open dating and what they mean. Remember that the dates are stamped at the processing plant and allow sufficient time for proper shipping, handling and home storage of the product:

* Sell-by (or pull) date: The last day a retailer is permitted (sometimes by law, sometimes by store policy) to sell a product as fresh. This type of date most often appears on perishables such as beef, fish, poultry, dairy products, bread and other bakery goods and some processed luncheon meats.

The sell-by date is determined by taking the entire shelf life of the product into consideration, including the time -- however subjective -- that the consumer may be expected to store it at home.

An example of how sell-by dates are determined, according to John Farquhar, vice president of scientific and technical services at the Food Marketing Institute: A salmon, with a shelf life of about 12 to 14 days at 29 to 30 degrees, is caught off Anchorage, Alaska, on Monday. It's processed Tuesday, iced and shipped on Wednesday and received in Houston on Friday.

The sell-by date stamped on the package by the processor, said Farquhar, would probably be the following Wednesday. That would be the 10th day the fish has been out of the water, allowing about a two-day storage life at home. Farquhar said the sell-by date is advanced two days as a built-in buffer against inadvertent temperature abuse or a slight delay in processing.

* Best-if-used-by date: Cereals, aseptically-packaged juices, snack foods, peanut butter, salad dressings, processed meats, cookies, refrigerated dough products and packaged yeasts carry this type of date. It signifies the period up to which the manufacturer determines the product will remain unchanged. There is, however, a division among manufacturers as to whether that guarantee applies once a package has been opened. In many cases, it depends on the type of product.

It doesn't mean that eating Tostitos two days after the best-if-used-by date will be a risk to your health. It refers to a quality perception, i.e. how long the product most likely will retain its flavor, aroma, texture and nutritional components.

Kellogg's, for instance, labels its cereals with "best-if-used-before" dates, according to company spokesperson Barbara Beck, to "reassure" the consumer that the cereal will remain at a certain standard if used within that time period. After a "reasonable" amount of time after that date, if stored and handled properly, the cereal is still safe to eat, said Beck, but the company cannot guarantee its quality.

* Best-if-purchased-by date: This method is used by at least a couple of manufacturers. Kraft, for instance, uses best-if-purchased-by on its Philadelphia Brand Cream Cheese, in which case the term means the same as sell-by.

* Expiration date: A once-popular dating method now in little use, although still found on egg cartons at Giant Food. It appears on the carton like this: EXP Jan 27, and means the same thing as the sell-by date -- the last day Giant can sell it. Safeway recently revamped its egg cartons, removing EXP and replacing it with a sell-by date.

And to cloud the situation some more, expiration dates also appear on over-the-counter drug labels, where they mean best-if-used-by.

Hold tight; EXP is not to be confused with EST, which appears on all USDA-inspected processsed meat (other than poultry) products. The USDA assigns every plant an establishment, or EST, number, which facilitates tracing of a product.

* Pack date: The date the food was packed or manufactured. This method is most frequently used on canned goods and other foods that have a long shelf life (e.g. oils, cake mixes). Such products are not really "open dated" since much of the time the package carries other information such as the plant number and/or the shift, all of which appear in a number-and-letter code unique to the individual company and usually undecipherable to the consumer. This information is used by the company in case it needs to locate a batch in the case of a product recall, for example.

An example of a translated pack date code: A cake mix packed by Pillsbury today would be marked with the code A5W27. The "A" means January (at Pillsbury, months are coded with letters, from A to L), the "5" is a shortcut for 1985, the "W" signifies the plant (at least to Pillsbury, which will not reveal its identity for competitive reasons) and the "27" is today's date.

To bring some consistency to open dating, the Food Marketing Institute, the trade association that represents the nation's supermarkets, issued a policy statement last year recommending that there be a voluntary uniform system of dating to straighten it all out.

FMI is suggesting that manufacturers stamp a sell-by date on packages as well as best-if-used-by information. For instance, it recommends, a yogurt carton label might state that the product is "best if used within 7 days after the sell-by date stamped on end of package." (Such a system is already on some foods; Safeway and Giant cardboard milk cartons, for example.)

"Consumers are frustrated and confused about the dates they find," said Odonna Mathews, consumer affairs adviser at Giant Food, who is working with FMI to spread the word about uniform open dating. In addition, said Mathews, the confusion spreads to store clerks who may be uncertain as to when to pull or restock an item.

Mathews said the dual system tells the consumer what he wants to know -- i.e. how long he can keep the product, as well as indicating to the supermarket how long it can sell the product. Mathews said in addition to milk, several Giant brand-name products now include both pieces of information -- granola bars, yogurt and tortilla chips, for example -- and that other labels are being phased in.

The Grocery Manufacturers of America, the trade association that represents manufacturers of supermarket goods, has not issued a recommendation to its members regarding the FMI statement, but instead has left it up to the discretion of the individual company.

But according to Mathews, because retailers are on the "front line," they hear about consumer confusion more frequently than manufacturers do. Manufacturers need to look at their policies and think of ways to improve them, she said.

Yet for some manufacturers and retailers, the FMI dual-dating system is not the way to go. For them, the situation gets murkier.

For example, since the 1960s, Safeway has been open dating all its perishable and semiperishable house brands with "sell-by" dates (the sell-by and best-if-used-by system on its milk is a recent test). Barbara Ettinger of Safeway said the reason the chain doesn't put "best-if-used-by" on a product is that the product goes "totally out of our hands once it leaves our store," a view reflected by other manufacturers reluctant to offer that information.

In other words, even though most manufacturers know the shelf lives of their products under certain test conditions, that shelf life may vary depending on the foods' storage and handling at home. Some manufacturers and retailers don't want to take that responsiblity.

Another reason that FMI's proposal has sparked controversy, said Ettinger, is the lack of consensus regarding how long products actually last. In fact, according to FMI's Farquhar, there is a "wide variation" between how different manufacturers of the same product determine best-if-used-by dates.

Even guides can conflict. A "Thumbs Up" brochure published by Giant Food, for instance, lists the shelf life of eggs at two to three weeks; the USDA publication "Your Money's Worth in Foods" lists the shelf life of eggs at five weeks.

And although Roger Coleman, spokesman for the National Food Processors Association, said that canned foods have a "indefinite shelf life, so long as the container is intact," the USDA publication "The Safe Food Book: Your Kitchen Guide" suggests that "to use canned foods wisely" consumers should use low-acid canned goods, such as canned meat and poultry, stored in a cabinet within two to five years and high-acid canned goods such as fruit juices and pickles within 12 to 18 months.

Bob Miller, chief of the canning procedures branch of the USDA's Food Safety and Inspection Service, said that while canned foods eaten after those dates may be perfectly safe, there may be quality changes. The objective of the "use wisely," said Miller, is to indicate to consumers that they should try to rotate canned products rather than store them for indefinite periods.

That brings up the whole issue of the appropriateness of open dates on such products as frozen food or canned goods, which have traditionally not been open dated because of their long shelf lives even though quality loss is obviously harder to gauge than with a smelly carton of outdated cottage cheese.

In fact, a supermarket industry source said that open dates have not been placed on most canned goods because they are long-term inventory products and companies may not want consumers to know how long the product has actually been sitting in a warehouse or supermarket shelf.

Coleman of the NFPA said that manufacturers and retailers "see to it" that there is an "orderly move through the distribution chain" so that processed foods go from manufacturer to consumer within a year (18 months being the outside limit).

Nevertheless, Coleman said food processors are moving toward voluntary open dating. Campbell's now puts a "best-if-used-by" date on the bottom of all its canned goods, and manufacturers of such other disparate products as Best Foods' Golden Griddle Maple Syrup and Vita Herring are doing the same.