When used in this Chapter:
(1) “Taxable year” means the calendar year, or the fiscal year ending during such calendar year, upon the basis of which the net income is computed under this Chapter. “Taxable year” includes, in the case of a return made for a fractional part of a year under the provisions of this Chapter or under regulations prescribed by the collector, the period for which return is made. The first tax as levied in this Chapter shall be for taxable years beginning after December 31, 1949, and shall be due in each case on the day next following the last day of each taxable year; and such tax shall be due annually thereafter, computed in each instance upon the net income of the calendar or fiscal year ended last preceding the date when the tax is due according to this Section. Due return and payment thereof shall be made as is elsewhere provided in this Chapter.
(2) “Fiscal year” means an accounting period of twelve months ending on the last day of any month other than December. However, no fiscal year will be recognized unless, before its close, it was definitely established as an accounting period by the taxpayer, and the books of such taxpayer were kept in accordance therewith; and no accounting period shall be changed without the approval of the collector.
(3) “Paid or incurred” and “paid or accrued” shall be construed according to the method of accounting upon the basis of which the net income is computed under this Chapter.