Section 684 - Interest on underpayment(a) General.-If any amount of income tax is not paid on or before the last date prescribed in this article for payment, interest on such amount at the underpayment rate set by the commissioner pursuant to section six hundred ninety-seven of this part, or if no rate is set, at the rate of seven and one-half percent per annum shall be paid for the period from such last date to the date paid, whether or not any extension of time for payment was granted. Interest under this subsection shall not be paid if the amount thereof is less than one dollar. If the time for filing of a return of tax withheld by an employer is extended, the employer shall pay interest for the period for which the extension is granted and may not charge such interest to the employee.(b) Exception as to estimated tax.-This section shall not apply to any failure to pay estimated tax.(c) Exception for mathematical or clerical error.-No interest shall be imposed on any underpayment of tax due solely to mathematical or clerical error if the taxpayer files a return within the time prescribed in this article (including any extension of time) and pays the amount of underpayment within three months after the due date of such return, as it may be extended.(d) Suspension of interest on deficiencies.-If a waiver of restrictions on assessment of a deficiency has been filed by the taxpayer, and if notice and demand by the tax commission for payment of such deficiency is not made within thirty days after the filing of such waiver, interest shall not be imposed on such deficiency for the period beginning immediately after such thirtieth day and ending with the date of notice and demand.(e) Tax reduced by carryback.-If the amount of tax for any taxable year is reduced by reason of a carryback of a net operating loss, such reduction in tax shall not affect the computation of interest under this section for the period ending with the filing date for the taxable year in which the net operating loss arises. Such filing date shall be determined without regard to extensions of time to file.(f) Interest treated as tax.-Interest under this section shall be paid upon notice and demand and shall be assessed, collected and paid in the same manner as income tax. Any reference in this article to the tax imposed by this article shall be deemed also to refer to interest imposed by this section on such tax.(g) Interest on penalties or additions to tax.-Interest shall be imposed under subsection (a) of this section in respect of any assessable penalty or addition to tax only if such assessable penalty or addition to tax is not paid within twenty-one calendar days from the date of the notice and demand therefor under subsection (b) of section six hundred ninety-two of this article (ten business days if the amount for which such notice and demand is made equals or exceeds one hundred thousand dollars), and in such case interest shall be imposed only for the period from such date of the notice and demand to the date of payment.(h) Payment within specified period after notice and demand.-If notice and demand is made for payment of any amount under subsection (b) of section six hundred ninety-two of this article, and if such amount is paid within twenty-one calendar days (ten business days if the amount for which such notice and demand is made equals or exceeds one hundred thousand dollars) after the date of such notice and demand, interest under this section on the amount so paid shall not be imposed for the period after the date of such notice and demand.(i) Limitation on assessment and collection.-Interest prescribed under this section may be assessed and collected at any time during the period within which the tax or other amount to which such interest relates may be assessed and collected, respectively.(j) Interest on erroneous refund.-Any portion of tax or other amount which has been erroneously refunded, and which is recoverable by the commissioner, shall bear interest at the underpayment rate set by the commissioner pursuant to section six hundred ninety-seven of this part, or if no rate is set, at the rate of seven and one-half percent per annum from the date of the payment of the refund, but only if it appears that any part of the refund was induced by fraud or a misrepresentation of a material fact.(k) Satisfaction by credits.-If any portion of a tax is satisfied by credit of an overpayment, then no interest shall be imposed under this section on the portion of the tax so satisfied for any period during which, if the credit had not been made, interest would have been allowable with respect to such overpayment.