Is there an expiry date for the aspirin?

Generally drug expiration dates can be viewed the same as 'Sell By' dates; they are a reference for when the product may no longer be 100%. In the case of drugs, that implies less effective. Actual shelf life will depend on storage conditions and the ingredients. Medications that are liquid or contain active ingedients will be more prone to degradation. Aspirin will usually remain effective a long while after the expiration date likely only with increasing ineffectiveness.