(a) Determination of credit—(1) In general. Section 667(b) allows under certain circumstances a credit (without interest) against the tax imposed by subtitle A of the Code on the beneficiary for the taxable year in which the accumulation distribution is required to be included in income under section 668(a). In the case of an accumulation distribution consisting only of undistributed net income, the amount of such credit is the total of the taxes deemed distributed to such beneficiary under section 666 (b) and (c) as a result of such accumulation distribution for preceding taxable years of the trust on the last day of which such beneficiary was in being, less the amount of such taxes for such preceding taxable years taken into account in reducing the amount of partial tax determined under § 1.668(b)-1A. In the case of an accumulation distribution consisting only of undistributed capital gain, the amount of such credit is the total of the taxes deemed distributed as a result of the accumulation distribution to such beneficiary under section 669 (d) and (e) for preceding taxable years of the trust on the last day of which such beneficiary was in being, less the amount of such taxes for such preceding taxable years taken into account in reducing the amount of partial tax determined under § 1.669(b)-1A. In the case of an accumulation distribution consisting of both undistributed net income and undistributed capital gain, a credit will not be available unless the total taxes deemed distributed to the beneficiary for all preceding taxable years as a result of the accumulation distribution exceeds the beneficiary's partial tax determined under §§ 1.668(b)-1A and 1.669(b)-1A without reference to the taxes deemed distributed. A credit is not allowed for any taxes deemed distributed as a result of an accumulation distribution to a beneficiary by reason of sections 666 (b) and (c) or sections 669 (d) and (e) for a preceding taxable year of the trust before the beneficiary was born or created. However, if as a result of an accumulation distribution the total taxes deemed distributed under sections 668(a)(2) and 668(a)(3) in preceding taxable years before the beneficiary was born or created exceed the partial taxes attributable to amounts deemed distributed in such years, such excess may be used to offset any liability for partial taxes attributable to amounts deemed distributed as a result of the same accumulation distribution in preceding taxable years after the beneficiary was born or created.

(2) Exact method. In the case of the tax computed under the exact method provided in §§ 1.668(b)-1A(b) and 1.669(b)-1A(b), the credit allowed by this section is computed as follows:

(i) Compute the total taxes deemed distributed under §§ 1.666(b)-1A and 1.666(c)-1A or §§ 1.669(d)-1A and 1.669(e)-1A, whichever are appropriate, for the preceding taxable years of the trust on the last day of which the beneficiary was in being.

(ii) Compute the total of the amounts of tax determined under § 1.668(b)-1A(b)(1) or § 1.669(b)-1A(b) (1), whichever is appropriate, for the prior taxable years of the beneficiary in which he was in being.

(3) Short-cut method. In the case of the tax computed under the short-cut method provided in § 1.668(b)-1A(c) or § 1.669(b)-1A(c), the credit allowed by this section is computed as follows:

(i) Compute the total taxes deemed distributed in all preceding taxable years of the trust under §§ 1.666(b)-1A and 1.666(c)-1A or §§ 1.669(d)-1A and 1.669(e)-1A, whichever are appropriate.

(ii) Compute the beneficiary's partial tax determined under either § 1.668(b)-1A(c)(1)(v) or § 1.669(b)-1A (c)(1)(v), whichever is appropriate.

(iii) Compute the total taxes deemed distributed under §§ 1.666(b)-1A and 1.666(c)-1A or §§ 1.669(d)-1A and 1.669(e)-1A, which are appropriate, for the preceding taxable years of the trust on the last day of which the beneficiary was in being.

(iv) Multiply the amount by which subdivision (i) of this subparagraph exceeds subdivision (ii) of this subparagraph by a fraction, the numerator of which is the amount determined under subdivision (iii) of this subparagraph and the denominator of which is the amount determined under subdivision (i) of this subparagraph. The result is the allowable credit. The application of this subparagraph may be illustrated by the following example:

(b) Year of credit. The credit to which a beneficiary is entitled under this section is allowed for the taxable year in which the accumulation distribution (to which the credit relates) is required to be included in the income of the beneficiary under section 668(a). Any excess over the total tax liability of the beneficiary for such year is treated as an overpayment of tax by the beneficiary. See section 6401(b) and the regulations thereunder.