(1) Interest on unpaid tax must be calculated as set forth in 15-1-216 , MCA.

(2) A taxpayer must file an amended Montana return within 90 days after filing an amended federal return or receiving notice that their taxable income was changed or corrected by the IRS, or other competent authority, as required by 15-30-304 , MCA.

(3) In the case where there is unpaid tax for a year that is reduced by the carryback of a subsequent year's net operating loss, interest on the unpaid tax accrues to the later of:

(a) the due date of the loss year return; or

(b) the date the loss year return is actually filed. This limited interest calculation applies only to the unpaid tax offset by the net operating loss carryback.