Here is a little info I found on Dow Chemical PG, I suspect it would be basically the same for any manufacture. Personally I don't think it actually goes bad and certainly not the day after it expires but the quality would start to diminish. In fact the quallity probably starts to diminish the day you open it.
PG USP/EP has a shelf life of two years when stored below 40°C in closed containers away from sources of UV light. An ongoing stability test program confirms this shelf life. The shelf life refers to the length of time during which a product complies with its sales specification when stored under appropriate conditions.
PG USP/EP is hygroscopic and may be subject to partial oxidation in the presence of oxygen. With increasing storage time, dependent on the storage conditions, the water content, color, odor and acidity can increase.