Section 43National Internal Revenue Code of 1997 (Republic Act No. 8424), Sec. 43

General Rule. - The taxable income shall be computed upon the basis of the taxpayer's annual accounting period (fiscal year or calendar year, as the case may be) in accordance with the method of accounting regularly employed in keeping the books of such taxpayer, but if no such method of accounting has been so employed, or if the method employed does not clearly reflect the income, the computation shall be made in accordance with such method as in the opinion of the Commissioner clearly reflects the income. If the taxpayer's annual accounting period is other than a fiscal year, as defined in Section 22(Q), or if the taxpayer has no annual accounting period, or does not keep books, or if the taxpayer is an individual, the taxable income shall be computed on the basis of the calendar year.