Mo. Rev. Stat. § 50.010

Unless otherwise provided in a charter adopted by a county under the provisions of Sections 18 or 31, 32 and 33 of Article VI, of the Constitution of this state, the fiscal year of the several counties of the state shall commence on January first and terminate on the thirty-first day of December in each year, and the books, accounts and reports of all county officers shall be made to conform thereto.