If date has expired but seal hasn't been opened, are pills still good?

For several decades, the program has found that the actual shelf life of many drugs is well beyond the original expiration dates.
A 2006 study of 122 drugs tested by the program showed that two-thirds of the expired medications were stable every time a lot was tested. Each of them had their expiration dates extended, on average, by more than four years, according to research published in the Journal of Pharmaceutical Sciences.
Some that failed to hold their potency include the common asthma inhalant albuterol, the topical rash spray diphenhydramine, and a local anesthetic made from lidocaine and epinephrine, the study said. But neither Cantrell nor Dr. Cathleen Clancy, associate medical director of National Capital Poison Center, a nonprofit organization affiliated with the George Washington University Medical Center, had heard of anyone being harmed by any expired drugs. Cantrell says there has been no recorded instance of such harm in medical literature.
Marc Young, a pharmacist who helped run the extension program from 2006 to 2009, says it has had a “ridiculous” return on investment. Each year the federal government saved $600 million to $800 million because it did not have to replace expired medication, he says.
An official with the Department of Defense, which maintains about $13.6 billion worth of drugs in its stockpile, says that in 2016 it cost $3.1 million to run the extension program, but it saved the department from replacing $2.1 billion in expired drugs. To put the magnitude of that return on investment into everyday terms: It’s like spending a dollar to save $677.

Some drugs have been found to maintain their potency for forty years!
Of course: There is no incentive from drug makers to do suitable tests so they can extend the expiry date.
So like all things in life, to those who are so sure they are Right... always think again.
cheers, Dr Luke