(a) Returns of taxpayers other than corporations shall be filed with the commissioner on or before April 15 in each year except that in the case of taxpayers using a fiscal year the return shall be filed on or before the fifteenth day of the fourth month after the close of the fiscal year. Returns of corporations made on the basis of a calendar year shall be filed on or before the fifteenth day of March following the close of the calendar year, and returns of corporations made on the basis of a fiscal year shall be filed on or before the fifteenth day of the third month following the close of the fiscal year. Returns required for a taxable year relating to returns of domestic import sales corporations and former domestic import sales corporations and foreign sales corporations shall be filed on or before the fifteenth day of the ninth month following the close of the taxable year. The commissioner may allow further time for filing returns in the case of sickness or other disability or whenever in his judgment good cause exists for the extension. In case a taxpayer is granted an extension of time to file a return, the commissioner may require a tentative return to be filed on or before the due date of the return for which the extension is granted. A tentative return shall be made on the usual form, shall be plainly marked "tentative," shall state the estimated amount of the tax believed to be due, and shall be properly signed by the taxpayer.

(b) A member of the armed forces of the United States serving outside the continental United States may file his required return for a taxable year ending during such service, without prior application, at any time within a period of six months following the return of the serviceman to the continental United States. During the period of extension, no interest shall accrue and no penalties shall be imposed.

(c) Any taxpayer may file an estimated income tax return within the taxpayer's taxable year in compliance with rules and regulations promulgated by the commissioner. Estimated returns shall be plainly marked "estimated."