(1) Unless otherwise provided by law, for all taxes, fees, and other assessments imposed under Titles 15 and 16, MCA, and administered by the department, interest attaches as outlined in 15-1-216, MCA.

(2) Exclusions to this rule are provided in 15-1-216, 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.

(4) Interest on all outstanding taxes shall accrue at the rate in effect, as provided in 15-1-216, MCA, during each calendar year, regardless of when the tax was originally due or when the tax was assessed. The rate does not affect any interest accrued prior to the current tax year.

(5) For purposes of determining interest on the underpayment of estimates provided in 15-30-2512, MCA, the rate in effect on the original due date of the tax return shall be used. For example, a return for the 2015 tax year is due April 15, 2016, so the rate that became effective January 1, 2016, shall be used to compute interest on the underpayment of estimates.

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