Does the “use by” date matter once the product is frozen?

No. When foods are frozen, they are kept safe indefinitely. Freezing food slows the motion of the molecules, causing microbes to become dormant. According to the USDA, freezing stops the growth of microorganisms—those that cause food spoilage and those that cause food-borne illnesses. But freezing does not kill all microbes. For example, as food scientist Susan Brewer points out, salmonella can survive for months in a frozen product and become active again when the product is thawed.
 
However, most consumers have noticed, the quality of some foods does not hold up indefinitely in the freezer. The texture and water content may change. Some bakery products kept in the freezer for a long time can develop a stale taste, though double-wrapping may keep them fresh longer. Ice cream in the freezer for months may develop ice crystals and/or turn gummy. Also, foods left frozen for many months may develop “freezer burn.” And, of course, some foods simply cannot be frozen because the texture would be spoiled—for example, raw eggs, lettuce, and mayonnaise.
 
It’s best to plan ahead and freeze a product while it’s at peak quality rather than near its expiration date.
 
Source(s):
USDA Fact Sheets “Safe Food Handling” “Freezing and Food Safety”
ConsumerReports.org Food “The dating game”
Susan Brewer, Ph.D., University of Illinois, Department of Food Science and Human Nutrition