26 CFR 1.1313(a)-1: (a)-1 Decision by Tax Court or other court as a determination.

(a) A determination may take the form of a decision by the Tax Court of the United States or a judgment, decree, or other order by any court of competent jurisdiction, which has become final.

(b) The date upon which a decision by the Tax Court becomes final is prescribed in section 7481.

(c) The date upon which a judgment of any other court becomes final must be determined upon the basis of the facts in the particular case. Ordinarily, a judgment of a United States district court becomes final upon the expiration of the time allowed for taking an appeal, if no such appeal is duly taken within such time; and a judgment of the United States Court of Claims becomes final upon the expiration of the time allowed for filing a petition for certiorari if no such petition is duly filed within such time.