Clif bar fan here and consumer of many out dated Clif bars, up to five years beyond evpiry date, as I recall.

The only thing I noticed was that the older bars seemed to be a bit drier than the younger ones. As far as I could tell, nutrition and energy benefits seemed to be unaltered. when bike touring, fully loaded style, my rule of thumb was that a Clif bar was good for twenty miles before another snack was needed..

My stash of quick chow is typically, but not always, stored in an even temperature, away from sunlight. I am pretty sure that storage conditions are very important in food storage.

"Expiration dates" are just guarantees that the product remains unaltered in any parameters of its condition -palatability, taste, nutrition, etc. No food producer does himself any favors by putting a longer, rather than a shorter, expiration date on a product.

I wouldn't worry very much about Clif bars that are beyond their dates. My experience is that it is not dangerous or unhealthy to eat out of date bars.