I work with Chemicals and the expiry date is set (eg. 1year) so that the chemical concentration is still within specifications. For example, Clomid at 50mg might be manufacted at 55mg +/- 2mg. After 1year the tabs are still gauranteed contain at least 50mg active ingredient. After 2year it may have dropped to 47mg so the manufacturer cannot set the expiry to greater than 1year.