Retrospective amendment of excise rules validates refusals of CENVAT credit and enables recovery within a fixed period with interest. The amendments to rules 57F and 57AB are deemed retrospective to dates in the Sixth Schedule, validating actions taken under the Central Excise Act during the intervening period as if the amended rules had always been in force; suits or enforcement to obtain credits not allowable under the deemed amendments are barred, credits improperly taken must be recovered within a prescribed period with interest for delayed payment, and the Central Government is deemed to have power to make such retrospective rules despite any supersession of earlier rules.