Praise be to Allah the Lord of the Worlds.
It is forbidden to tamper with the expiry date of a product, because doing so is an act of deception, lying and fraud. Allah The Almighty Says: "O ye who believe! Fear God and be with those who are true (in word and deed)."{At-Tawba, 119}. In addition, the Messenger of Allah (PBUH) says: "Whoever cheats us isn`t one of us." {Related by Muslim No.101}.
Moreover, tampering with expiry dates is harmful to the health of consumers since all food products have an expiry date and can`t remain edible forever. In fact, leniency in this serious matter will definitely inflict public harm although the Messenger of Allah (PBUH) say: "There should be neither harm nor reciprocating harm." {A sound narration /Ibn Majah}. And Allah the Almighty Knows Best.