I started researching for you and found this. I didn't think of expiration dates and the fact that alcohol evaporates creating a less potent formula than what's noted on the label.
"To work, though, research has shown that these products must consist of at least 60 percent alcohol. If the alcohol concentration is any lower, the sanitizer will not kill off all the germs. In fact, studies seem to show that faulty gels — ones with less than this amount of alcohol — seem to mobilize the germs, spreading them around the hand instead of killing them."
"...alcohol is a highly volatile liquid. That means that it evaporates quickly when exposed to the air. Although the sanitizer remains in a container where exposure is minimal, it's not airtight. So the alcohol in the gel will evaporate."
Does hand sanitizer really expire?