The amount of interest tax and the amount of delinquency additional tax that was to be paid or collected pursuant to the provisions of the former tax laws with respect to national tax imposed or to be imposed before the enforcement of this Act (with regard to the amount of delinquency additional tax, limited to that pertaining to national tax for which a demand has been made pursuant to the provisions of the former tax laws before the enforcement of this Act) are governed by the former tax laws; provided, however, that the period used in calculating these amounts is to end on the day preceding the date of enforcement of this Act.