The interest upon the certificate provided in Section 6467 shall be payable semiannually, by coupon, on the second day of December and June, respectively, of each year a principal payment matures. The certificate shall bear date determined by the date of bonds if bonds are to be issued in the proceedings. If no bonds are to be issued the certificate or certificates shall bear such date as may be determined by the legislative body conducting the proceedings. The rate of interest shall not exceed 7 percent per annum on all unpaid sums.