Hearings upon the formation of an assessment district, upon a change of organization for an existing district, or upon the levy of annual assessments after formation of a district shall be concluded and any resolution confirming a diagram and an assessment shall be adopted not later than:
(a) July 1 of the fiscal year during which the assessments are to be collected on the county assessment roll; or
(b) Such later date, not beyond the third Monday in August, as the county auditor may authorize.