If it stinks--it's bad. "cook by" dates are not always accurate, and you have to remember the length of processing time from "farm" to store.
Also, it is likely older than you think, because most chicken has already been frozen for transport and storage. So, you are buying a pre-frozen and thawed piece of meat. This means it cannot be re-frozen prior to cooking it in the first place.
But, consider this: would you rather throw it away, and lose a few bucks, or eat it and end up very sick and possibly in the hospital?
Throw that chicken away, and fast!