Vitamin C is also known as ascorbic acid or ascorbate. It can undergo a type of chemical reaction known as oxidation, which changes its chemical form. The oxidized product of vitamin C is known as dehydroascorbic acid, or DHAA. Although this compound can be converted back into vitamin C, that chemical reaction does not occur naturally.
Although DHAA isn't toxic, it does not function effectively as vitamin C, meaning that over time, vitamin C supplements become less potent. However, different vitamin C supplements turn into DHAA at different rates. One factor that controls this is the initial quality of the supplement. Supplements that are packaged with very little moisture and are mostly free of any oxidized compounds will break down more slowly than poor quality supplements. However, there are no large-scale studies that have tested the degradation of different kinds of vitamin C, so it is not known which brands are the "best" in terms of expiration date.