(1) such tax, or the balance thereof, shall be payable to the commissioner of finance in full at the time its return is required to be filed, and
(2) such tax, or the balance thereof, imposed on any taxpayer which ceased to exercise its franchise or to be subject to the tax imposed by this part shall be payable to the commissioner of finance at the time the return is required to be filed, provided such tax of a domestic corporation which continues to possess its franchise shall be subject to adjustment as the circumstances may require; all other taxes of any such taxpayer, which pursuant to the foregoing provisions of this subdivision would otherwise be payable subsequent to the time such return is required to be filed, shall nevertheless be payable at such time.
(b) If the taxpayer, within the time prescribed by subdivision (c) of section 11-646 of this part, shall have applied for an automatic extension of time to file its annual return and shall have paid to the commissioner of finance on or before the date of such application is filed an amount properly estimated as provided by said subdivision the only amount payable in addition to the tax shall be interest at the underpayment rate set by the commissioner of finance pursuant to section 11-687 of this chapter, or, if no rate is set, at the rate of seven and one-half percent per annum upon the amount by which the tax, or portion thereof payable on or before the date the return was required to be filed, exceeds the amount so paid. For the purposes of the preceding sentence:
(1) an amount so paid shall be deemed properly estimated if it is either:
(i) not less than ninety per cent of the tax as finally determined, or
(ii) not less than the tax shown on the taxpayer's return for the preceding taxable year, if such preceding year was a taxable year of twelve months; and
(2) the time when a return is required to be filed shall be determined without regard to any extension of time for filing such return.
(c) The commissioner of finance may grant a reasonable extension of time for payment of any tax imposed by this part under such conditions as the commissioner deems just and proper.