Section 21. An authority is authorized to provide by resolution at one time or from time to time for the issue of interest bearing or discounted notes for the purposes and in the amounts that bonds may be issued. The notes shall be payable within three years from their dates, but the principal of and interest on notes issued for a shorter period may be renewed or paid from time to time by the issue of other notes hereunder maturing within the required time from the date of the original loan being refunded. When bonds are issued for the purposes for which the notes were issued, the proceeds of the bonds shall be used to repay the notes, except that interest on the notes may be financed as a current expense to the extent deemed appropriate by the authority. The notes may be secured by a trust agreement or by the provisions of a resolution, as in the case of bonds. Bond anticipation notes may be issued either before or after the authorization of the bonds being anticipated. If any bond anticipation note is paid otherwise than from the proceeds of bonds or renewal notes, such payment shall be included in the measure of the net cost of service. But, if bonds or renewal notes are later issued to provide for such payment, there shall be a corresponding offset against the net cost of service.