Conn. Gen. Stat. § 12-146
Unless the context otherwise requires, wherever used in this section, "tax" includes each property tax and each installment and part thereof due to a municipality as it may have been increased by interest, fees and charges. If any tax due in a single installment or if any installment of any tax due in two or more installments is not paid in full (1) on or before the first day of the month next succeeding the month in which it became due and payable, or if not due and payable on the first day of the month, (2) on or before the same date of the next succeeding month corresponding to that of the month on which it became due and payable, the whole or such part of such installment as is unpaid shall thereupon be delinquent and shall be subject to interest from the due date of such delinquent installment. Except for unpaid real estate taxes the collection of which was, or is, deferred under the provisions of section 12-174, and any predecessor and successor thereto, which unpaid real estate taxes continue to be subject to the provisions of such deferred collection statutes, the delinquent portion of the principal of any tax shall be subject to interest at the rate of eighteen per cent per annum from the time when it became due and payable until the same is paid, subject to a minimum interest charge of two dollars per installment which any municipality, by vote of its legislative body, may elect not to impose, and provided, in any computation of such interest, under any provision of this section, each fractional part of a month in which any portion of the principal of such tax remains unpaid shall be considered to be equivalent to a whole month. Each addition of interest shall become, and shall be collectible as, a part of such tax. Interest shall accrue at said rate until payment of such taxes due notwithstanding the entry of any judgment in favor of the municipality against the taxpayer or the property of the taxpayer. The collector shall apply each partial payment to the wiping out of such interest before making any application thereof to the reduction of such principal. If any tax, at the time of assessment or because of a subsequent division, represents two or more items of property, the collector may receive payment in full of such part of the principal and interest of such tax as represents one or more of such items, even though interest in full on the entire amount of the principal of such tax has not been received up to the date of such payment; in which event, interest on the remaining portion of the principal of any such tax shall be computed, as the case may be, from the due date of such tax if no other payment after delinquency has been made or from the last date of payment of interest in full on the whole amount or unpaid balance of the principal of such delinquent tax if previous payment of interest has been made. Each collector shall keep a separate account of such interest and the time when the same has been received and shall pay over the same to the treasurer of the municipality of the collector as a part of such tax. No tax or installment thereof shall be construed to be delinquent under the provisions of this section if (A) such tax or installment was paid through a municipal electronic payment service within the time allowed by statute for payment of such tax or installment, or (B) the envelope containing the amount due as such tax or installment, as received by the tax collector of the municipality to which such tax is payable, bears a postmark showing a date within the time allowed by statute for the payment of such tax or installment. Any municipality may, by vote of its legislative body, require that any delinquent property taxes shall be paid only in cash or by certified check or money order. Any municipality adopting such requirement may provide that such requirement shall only be applicable to delinquency exceeding a certain period in duration as determined by such municipality. Any municipality shall waive all or a portion of the interest due and payable under this section on a delinquent tax with respect to a taxpayer who has received compensation under chapter 968 as a crime victim.
Conn. Gen. Stat. § 12-146
(1949 Rev., S. 1828; 1969, P.A. 54, S. 2; P.A. 73-494, S. 1, 2; 73-508; P.A. 74-247, S. 1, 2; P.A. 75-296, S. 1, 3; P.A. 80-468, S. 2, 3; P.A. 81-44, S. 2, 3; P.A. 82-141, S. 2, 4; P.A. 83-361, S. 1, 3; P.A. 84-257, S. 1, 2; 84-282, S. 3; P.A. 85-286, S. 1, 2; P.A. 99-128, S. 2; P.A. 00-200, S. 2; June 30 Sp. Sess. P.A. 03-6, S. 58; P.A. 04-126, S. 1; P.A. 13-276, S. 23; P.A. 15-156, S. 2.)
Taxes carry interest only by statute. 67 C. 162. Appeal does not suspend running of interest. 72 C. 599. Cited. 123 Conn. 548. Rate of interest under section is compensatory and not punitive. 45 CS 283.
See Sec. 1-2a re construction of references to "United States mail", "postmark" or "registered or certified mail".