Expiration dates on most things from food to drugs are driven entirely by liability not efficacy. In other words, determined by lawyers.
Two observations. One, Americans throw away more food based on a spurious date than most of the world eats. Two, on my prescription bottes the pharmacy puts a use by date. This date is always one year after purchase. Often times they cover the manufacturer's expiration date. The manufacturer's dates have always been considerably longer. But you can always get the manufacturer's date by contacting them with the lot number. In fact, they are required to provide that information.
I actually did get some insulin once where the pharmacy use by date was almost a year after the manufacturer's use by date. The first attempt at deflection was for the pharmacy to claim that "drugs don't expire exactly on the date on the package". When I mentioned I would just contact the manufacturer to check the truth of this claim amazing things happened. The pharmacist freaked out and contacted his corporate folks.
Turned out the corporate folks were required to report the incident to the government.