No bonds issued under the provisions of this chapter may bear interest at a rate or rates and be sold privately at a price resulting in an average net interest cost higher than twelve percent per annum. There is no interest rate ceiling on those issues sold at public sale or to the state of North Dakota or any of its agencies or instrumentalities. No bonds issued under this chapter may run for a longer period than twenty years from their date. The bonds may not bear a date earlier than the date of the election authorizing their issuance, if such election is required, nor earlier than the date of the adoption of the resolution of the governing body determining to issue bonds for which no election is required.