All past due receivable accounts shall be subject to an interest charge of 0.87 percent per month or 10.5 percent per annum. The interest charge will be assessed on all outstanding receivable accounts that have been determined to be 60 days past due.

Any written agreements made prior to the passage of the ordinance codified in this chapter shall not be affected unless otherwise stated in the agreement(s). [Code 1979 § 6.04.]