For a taxpayer who uses the accrual method, income and expenses relating to a long term contract shall be calculated on the basis of the percentage of the work completed during the taxable year.

The percentage of work completed shall be determined by comparing the costs of the contract incurred during the taxable year with the total estimated cost of the contract.

For the purposes of this Article, the term 'long term contract' shall mean a contract for the manufacture, installation, construction, or performance of services related thereto, and whose execution is not completed within the year in which execution started, with the exception of a contract that is expected to be completed within six months of the actual starting date of work cited in the contract.