Mont. Code Ann. § 7-2-2806

If, upon the canvass of votes cast at the election, more than 50% of the votes cast in each adjoining county approve the proposed boundary change, then the boards of county commissioners in the adjoining counties shall, by resolution, declare the boundary to be changed as of January 1 of the year that begins at least 13 months after the date the election is held and shall direct the transfer of all certified copies of property records and other records to the appropriate county to be completed by the date the boundary change becomes effective. The resolution must include the legal description of the new boundaries of each county.