For sure, they will not improve over time, and minor quality loss may occur. But most importantly, canned and dried foods will remain completely safe, provided the packages are intact and they are stored at normal conditions.
Brownie mixes usually have about a one-year best-by date. I assure you, however, they will be good for many more months past that date.
Unfortunately, over-reliance on these dates means that retailers and consumers often toss out perfectly safe and wholesome foods. In times of want for many Americans, it’s a shame that as much as 30% of the U.S. food supply is discarded.
There are some exceptions where use-by dates are more meaningful. For example, regular white flour remains fine for up to a year. But whole wheat flour can become stale or rancid in just a few months.
You will also find use-by dates on frozen foods. They will remain safe, as long as they stay frozen. But quality will deteriorate over time. This is especially true for foods held in self-defrosting freezers, where short increases in temperature normally occur to melt off frost build-up
When the temperature returns to freezing, large ice crystals will form in foods. These crystals are particularly damaging to meat and fish. Processed vegetables and frozen dinners, especially those that contain sauces, may also suffer quality defects.