The provisions of this ordinance shall not apply to any construction project for which a valid zoning permit has been issued prior to the enactment of this ordinance, with the provision, however, that construction, has substantially commenced as defined in this ordinance, within six months after the issuance of the permit and with the further provision, however, that in the case of permits granted in the Shoreland-Floodplain District that substantial construction is completed, as defined in this ordinance, within 15 months after the issuance of the permit. [Code § 12.02-5.]