All products have a before and after date on the package. It is very important to keep track of these dates because in some products it can be critical. Bread yeast is a living organism and can lose its potency. There is a possibility that the dough may not rise at all or it may take much longer than stated in the recipe. To know if your yeast has expired - add a small amount of yeast in warm water and let stand for 5 minutes. If it forms up - you are good to go. If not, toss it out and use a new package of yeast.