(1) When a taxpayer's federal taxable income is changed or corrected by the Internal Revenue Service, or other authority, the taxpayer must file an amended return, Montana Form 2X, within 90 days after receiving notice from the IRS.

(2) When a taxpayer changes his or her own federal taxable income by amending his or her federal income tax return, the taxpayer must file an amended return, Montana Form 2X, reporting these changes within 90 days after filing the federal amended tax return.

(3) Interest and penalty are assessed on any unpaid tax under the original return from the prescribed due date of the original return until the department receives payment.