Typically two years is all any legal company will insure their products. Just because it's past the date though does not mean it's gone bad or anything. There is a slight breakdown in the potency after time, but that is less of a worry with solid orals, slightly more likely for injectables/liquids. (They react to light and temp more easily). Honestly the expert ion date put on most products is one that they know for sure the product still has its full integrity. They often stop testing after a couple years because it's a waste of money for them to continue testing after a certain point. Truth is that many pharmaceuticals are still highly effective long after their expiration dates.
(I only know this because I used to work in pharmaceutical manufacturing and we also did a lot of the testing for FDA approvals)