From Hershey's website FAQ...
'Q. How long should I keep chocolate?
A. Most confectionery products are at their best flavor for one year after manufacture. Ingredients such as nuts will shorten the shelf life. Products kept beyond recommended "best before" date may have flavor loss or texture changes. Keep in mind, however, that storage conditions greatly affect the quality of our products.
Q. My chocolate sometimes turns tan or white. What causes this?
A. Chocolate contains cocoa butter, a vegetable fat that is sensitive to heat and humidity. Temperatures above 75�F will cause chocolate to melt. The cocoa butter can rise to the surface and form a discoloration called "cocoa butter bloom." Condensation on milk or semi-sweet chocolate may cause the sugar to dissolve and rise to the surface as "sugar bloom." Chocolate that has "bloomed" is certainly safe to use, but flavor loss and texture changes may be noticed.