(a) after May thirty-first and before September first of such current taxable year, the declaration shall be filed on or before September fifteenth, or

(b) after August thirty-first and before December first of such current taxable year, the declaration shall be filed on or before December fifteenth. 4. A taxpayer may amend a declaration of estimated tax or a declaration of estimated tax surcharge under regulations prescribed by the commissioner of taxation and finance. 5. If, on or before February fifteenth of the succeeding year in the case of a taxpayer whose taxable year is a calendar year, a taxpayer files its return for the year for which the declaration is required, and pays therewith the balance, if any, of the full amount of the tax or tax surcharge shown to be due on the return:

(a) such return shall be considered as its declaration if no declaration was required to be filed during the taxable year for which the tax or tax surcharge was imposed, but is otherwise required to be filed on or before December fifteenth pursuant to paragraph (b) of subdivision three of this section, and

(b) such return shall be considered as the amendment permitted by subdivision four of this section to be filed on or before December fifteenth if the tax or tax surcharge shown on the return is greater than the estimated tax or estimated tax surcharge, as the case may be, shown on a declaration previously made. 6. If the taxable period for which a tax or tax surcharge is imposed under any of the sections enumerated in subdivision one hereof is other than twelve months, every taxpayer required to make a declaration of estimated tax or a declaration of estimated tax surcharge for such taxable period shall make such declaration in accordance with rules of the commissioner. 7. This section shall apply to taxable periods of twelve months other than a calendar year by the substitution of the months of such fiscal year for the corresponding months specified in this section. 8. The tax commission may grant a reasonable extension of time, not to exceed three months, for the filing of any declaration required pursuant to this section, on such terms and conditions as it may require.