(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, penalty and interest attaches as outlined in 15-1-216, MCA.

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

(3) Interest on all outstanding individual income taxes after December 31, 2007, shall accrue at the rate of 8% per annum calculated daily. The rate applies to all outstanding taxes regardless of the tax year or when the return was filed. The rate does not affect any interest accrued prior to December 31, 2007. Examples of how this rule affects the calculation of interest are as follows:

(a) Taxpayer A timely filed a return for the 2005 tax year but has not paid their liability. From the original due date of April 15, 2006, until December 31, 2006, interest is assessed at 12% per annum. From January 1, 2007, until December 31, 2007, interest is assessed at 8% per annum. From January 1, 2008, until December 31, 2008, interest is assessed at 8% per annum.

(b) Taxpayer B files a return for the 2004 tax year on March 15, 2008, and pays the tax at that time. From the original due date of April 15, 2005, until December 31, 2006, interest is assessed at 12% per annum. From January 1, 2007, until December 31, 2007, interest is assessed at 8% per annum. From January 1, 2008, until March 15, 2008, interest is assessed at 8% per annum.

(c) Taxpayer C files a return for the 2005 tax year on October 15, 2006, and pays the tax and interest at a rate of 12% at that time. On June 7, 2008, the taxpayer files an amendment to the 2005 tax year to report an increase in tax resulting from a federal revenue agent's report. Interest on the increased liability is computed as follows: From the original due date of April 15, 2006, until December 31, 2006, interest is assessed at 12% per annum. From January 1, 2007, until December 31, 2007, interest is assessed at 8% per annum. From January 1, 2008, until December 31, 2008, interest is assessed at 8% per annum.

(4) For purposes of determining interest on the underpayment of estimates provided in 15-30-241, MCA, the rate in effect on the original due date of the tax return shall be used. For example, a return for the 2007 tax year is due April 15, 2008, so the rate that became effective January 1, 2008, shall be used to compute interest on the underpayment of estimates.