Section 48National Internal Revenue Code of 1997 (Republic Act No. 8424), Sec. 48

Accounting for Long-term Contracts. - Income from long-term contracts shall be reported for tax purposes in the manner as provided in this Section. As used herein, the term 'long-term contracts' means building, installation or construction contracts covering a period in excess of one (1) year. Persons whose gross income is derived in whole or in part from such contracts shall report such income upon the basis of percentage of completion. The return should be accompanied by a return certificate of architects or engineers showing the percentage of completion during the taxable year of the entire work performed under contract.

There should be deducted from such gross income all expenditures made during the taxable year on account of the contract, account being taken of the material and supplies on hand at the beginning and end of the taxable period for use in connection with the work under the contract but not yet so applied. If upon completion of a contract, it is found that the taxable net income arising thereunder has not been clearly reflected for any year or years, the Commissioner may permit or require an amended return.