(a) In computing the amount of the adjustment under sections 1311 to 1315, inclusive, there must first be ascertained the amount of the tax previously determined for the taxpayer as to whom the error was made for the taxable year with respect to which the error was made. The tax previously determined for any taxable year may be the amount of tax shown on the taxpayer's return, but if any changes in that amount have been made, they must be taken into account. In such cases, the tax previously determined will be the sum of the amount shown as the tax by the taxpayer upon his return and the amounts previously assessed (or collected without assessment) as deficiencies, reduced by the amount of any rebates made. The amount shown as the tax by the taxpayer upon his return and the amount of any rebates or deficiencies shall be determined in accordance with the provisions of section 6211 and the regulations thereunder.
(b)(1) The tax previously determined may consist of tax for any taxable year beginning after December 31, 1931, imposed by subtitle A of the Internal Revenue Code of 1954, by chapter 1 and subchapters A, B, D, and E of chapter 2 of the Internal Revenue Code of 1939, or by the corresponding provisions of prior internal revenue laws, or by any one or more of such provisions.
(2) After the tax previously determined has been ascertained, a recomputation must then be made under the laws applicable to said taxable year to ascertain the increase or decrease in tax, if any, resulting from the correction of the error. The difference between the tax previously determined and the tax as recomputed after correction of the error will be the amount of the adjustment.
(c) No change shall be made in the treatment given any item upon which the tax previously determined was based other than in the correction of the item or items with respect to which the error was made. However, due regard shall be given to the effect that such correction may have on the computation of gross income, taxable income, and other matters under chapter 1 of the Code. If the treatment of any item upon which the tax previously determined was based, or if the application of any provisions of the internal revenue laws with respect to such tax, depends upon the amount of income (e.g. charitable contributions, foreign tax credit, dividends received credit, medical expenses, and percentage depletion), readjustment in these particulars will be necessary as part of the recomputation in conformity with the change in the amount of the income which results from the correct treatment of the item or items in respect of which the error was made.
(d) Any interest or additions to the tax collected as a result of the error shall be taken into account in determining the amount of the adjustment.
(e) The application of this section may be illustrated by the following example:
Example:(1) For the taxable year 1949 a taxpayer with no dependents, who kept his books on the cash receipts and disbursements method, filed a joint return with his wife disclosing adjusted gross income of $42,000 deductions amounting to $12,000, and a net income of $30,000. Included among other items in the gross income were salary in the amount of $15,000 and rents accrued but not yet received in the amount of $5,000. During the taxable year he donated $10,000 to the American Red Cross and in his return claimed a deduction of $6,300 on account thereof, representing the maximum deduction allowable under the 15-percent limitation imposed by section 23(o) of the Internal Revenue Code of 1939 as applicable to the year 1949. In computing his net income he omitted interest income amounting to $6,000 and neglected to take a deduction for interest paid in the amount of $4,500. The return disclosed a tax liability of $7,788, which was assessed and paid. After the expiration of the period of limitations upon the assessment of a deficiency or the allowance of a refund for 1949, the Commissioner included the item of rental income amounting to $5,000 in the taxpayer's gross income for the year 1950 and asserted a deficiency for that year. As a result of a final decision of the Tax Court of the United States in 1955 sustaining the deficiency for 1950, an adjustment is authorized for the year 1949.
(2) The amount of the adjustment is computed as follows:
| Tax previously determined for 1949 | $7,788
|
| Net income for 1949 upon which tax previously determined was based | 30,000
|
| Less: Rents erroneously included | 5,000
|
| Balance | 25,000
|
| Adjustment for contributions (add 15 percent of $5,000) | 750
|
| Net income as adjusted | 25,750
|
| Tax as recomputed | 6,152
|
| Tax previously determined | 7,788
|
| Difference | 1,636
|
| Amount of adjustment to be refunded or credited | 1,636 |