Mont. Code § 7-3-154
Judicial review to determine the validity of the procedures whereby any charter or alternative plan of government is adopted may be initiated by petition in district court of 10 or more registered voters of the local government brought within 60 days after the election at which the charter or plan of government, revision, or amendment is approved. If no petition is filed within that period, compliance with all the procedures required by 7-3-121 through 7-3-123, 7-3-125, and 7-3-141 through 7-3-161 and the validity of the manner in which the charter or plan of government was approved is conclusively presumed. It is presumed that proper procedure was followed and all procedural requirements were met. The adoption of a charter or plan of government may not be considered invalid because of any procedural error or omission unless it is shown that the error or omission materially and substantially affected its adoption.
§ 7-3-154, MCA