N.J. Stat. Ann. § 26:4-49

In case a defendant shall have been twice convicted, within the space of six months, of the violation of the same provision of this article and due proof of such fact is made, the court may, in addition to the imposition of the penalty prescribed by section 26:4-129 of this Title, cause the defendant to be imprisoned, with or without hard labor, for any number of days not exceeding one for each dollar of the penalty.