N.J. Stat. Ann. § 17:3B-32

4. A bank may charge and collect periodic interest under a revolving credit plan on outstanding unpaid indebtedness in the borrower's account under the plan at such daily, weekly, monthly, annual or other periodic percentage rate or rates as the agreement governing the plan provides or as established in the manner provided in the agreement governing the plan. If the applicable periodic percentage rate under the agreement governing the plan is other than daily, periodic interest may be calculated on an amount not in excess of the average outstanding unpaid indebtedness for the applicable billing period, determined by dividing the total of the amounts of outstanding unpaid indebtedness for each day in the applicable billing period by the number of days in the billing period. If the applicable periodic percentage rate under the agreement governing the plan is monthly, a billing period shall be deemed to be a month or monthly if the last day of each billing period is on the same day of each month or does not vary by more than four days therefrom.