Mont. Code Ann. § 15-31-506

Every corporation shall, upon request of the department of revenue, furnish a copy of its federal income tax return and the computation schedule filed for the taxable year or years that the department may specify in its request. Except as provided in 15-30-3403 or 15-30-3404, if the amount of a corporation's taxable income reported on its federal income tax return or the computation schedule filed for a tax year is changed or corrected by the United States internal revenue service or other competent authority, the corporation shall file an amended Montana return with the department within 180 days after the final determination date. Except as provided in 15-30-3404, a corporation filing an amended federal income tax return changing or correcting its taxable income for a tax year shall also file an amended Montana return with the department within 180 days after filing an amended federal income tax return.