Bonds shall bear dates prescribed by the board. Bonds may be serial bonds or sinking fund bonds with such maturities as the board may determine. No bond by its terms shall mature in more than 50 years from its own date and, in the event any authorized issue is divided into two or more series or divisions, the maximum maturity date authorized by this section shall be calculated from the date on the face of each bond separately, irrespective of the fact that different dates may be prescribed for the bonds of each separate series or division of any authorized issue.