Mont. Code Ann. § 20-6-412

The property tax valuation used under the provisions of 20-9-142 for the purposes of fixing the tax levies, except the debt service fund tax levy, for a district that has had a boundary change at any time before the fourth Monday in August shall include the property tax valuation of any territory added to the district by such boundary change or exclude the property tax valuation of any territory detached from the district by such boundary change.