The expiration date stamped on the Old El Paso taco shell package reflects the manufacturer's suggested amount of time for ultimate quality of the shells. The shelf life, however, is usually much longer, although the extended time could affect quality and freshness.
The ultimate shelf life of Old El Paso taco shells depends on a variety of circumstances, including storage options and whether the internal packaging has been opened. If the shells remain in the sealed plastic inside the box, they are most likely safe for consumption long past the expiration date.
If consuming after the expiration date, carefully inspect the shells to ensure they are safe to eat. If they have developed an odd odor or appearance, or if mold appears, do not eat the shells; instead, discard them.
The "best if used by" date usually stamped on food products generally represents the manufacturers' estimate of the best length of time by which the product will remain at peak quality.