Conn. Gen. Stat. § 12-405c

(b) The commissioner may for reasonable cause extend the time for the filing of any return due under this chapter and the payment of tax due thereon under such rules and regulations as he may prescribe. Said commissioner may require the filing of a tentative return and the payment of an estimated tax. Any additional tax which may be found to be due on the filing of a final return as allowed by such extension shall bear interest at the rate of one and one-fourth per cent per month or fraction thereof from the original due date of such tax to the date of actual payment.

(c) Any taxpayer whose taxable income for federal income tax purposes is adjusted or corrected for any taxable year or portion thereof by any official of the United States government, or any agency thereof, in any respect affecting the tax imposed under this chapter shall, within ninety days after having received written notification of such adjustment or correction, submit to the Commissioner of Revenue Services an affidavit disclosing such changes or adjustments and thereafter shall promptly furnish to the commissioner any information, schedules, records, documents or papers related to such change, adjustment or correction as he may require. Any taxpayer whose return to the Director of Internal Revenue has been amended in any respect affecting the tax imposed under this chapter shall, within ninety days after having filed such amended return, make an amended return to the commissioner. The time for filing such affidavit or amended return may be extended by the commissioner upon due cause shown. Notwithstanding the limitation of time in subsection (d) of section 12-405d , if, upon examination, the commissioner finds that such taxpayer is liable for the payment of an additional tax, he shall, within a reasonable time from the receipt of an amended return or affidavit, notify such taxpayer of the amount of such additional tax, together with interest thereon computed at the rate of one and one-fourth per cent per month or any fraction thereof from the date when the original tax became due and payable. Within thirty days of the mailing of such notice, the taxpayer shall pay to the commissioner the amount of such additional tax and interest. If, upon examination of such amended return or affidavit and related information, the commissioner finds that the taxpayer has overpaid the tax due the state, the commissioner shall certify the amount of such overpayment to the Comptroller, and such amount shall be paid to the taxpayer by the State Treasurer upon order of the Comptroller.