All bills for service shall be rendered monthly as of the first day of the month succeeding the period for which the service is billed (each period of service or “service month” shall be 30 or 31 days from the twentieth of one calendar month to the twentieth of the next following calendar month), and shall be payable not later than the close of business on the twentieth day of the same month.
(2009 Code, § 39-1-17)