26 U.S.C. § 7481 — Date when Tax Court decision becomes final

(a) Reviewable decisions: Except as (b), (c), and (d) provide, a Tax Court^* decision becomes final:
(1) Timely notice of appeal not filed: once the time allowed for filing a notice of appeal runs out, if no one filed one.
(2) Decision affirmed or appeal dismissed: (A) once the time for filing a petition for certiorari (asking the Supreme Court to review) runs out, if the Court of Appeals affirmed the Tax Court or dismissed the appeal and no one filed that petition; (B) once the Supreme Court denies a certiorari petition, under the same circumstances; or (C) 30 days after the Supreme Court issues its mandate, if the Supreme Court orders the decision affirmed or the appeal dismissed.
(3) Decision modified or reversed: (A) if the Supreme Court orders the decision modified or reversed, the Tax Court's decision on remand becomes final 30 days after it is issued - unless within those 30 days the IRS or the taxpayer^* starts a proceeding to correct it to match the mandate, in which case it becomes final once corrected. (B) If the Court of Appeals modifies or reverses the decision, and either the time for a certiorari petition ran out with none filed, or the petition was denied, or the Supreme Court affirmed the Court of Appeals, then the Tax Court's decision on remand becomes final 30 days after being issued, unless corrected the same way as above.
(4) Rehearing: if the Supreme Court orders a rehearing, or the Court of Appeals sends the case back to the Tax Court for one, and the same three conditions in (3)(B) are met, then the Tax Court's new decision after rehearing becomes final the same way as if there had been no earlier decision.
(5) Definition of "mandate": in this section, "mandate" means the final mandate, even if an earlier one was recalled within 30 days of being issued.

(b) Nonreviewable decisions: a Tax Court decision in a case handled under section 7436(c) or 7463 becomes final 90 days after it is entered.

(c) Jurisdiction over interest determinations:
(1) In general: despite subsection (a), if within one year after a decision becomes final, the taxpayer files a motion asking the Tax Court to redetermine the interest amount, the Tax Court can reopen the case just to decide whether the taxpayer overpaid interest or the IRS underpaid interest, and by how much.
(2) This applies where (A)(i) the IRS made an assessment under section 6215 that includes interest, and (ii) the taxpayer paid the full deficiency^* plus the interest the IRS claimed, and (B) the Tax Court finds under section 6512(b) that the taxpayer overpaid.
(3) Special rules: if the Tax Court finds an overpayment or underpayment of interest this way, that finding counts under section 6512(b)(1) as a determination of an overpaid tax. An order redetermining interest, once entered in court records, can be reviewed the same way as a regular Tax Court decision.

(d) Decisions relating to estate tax extended under section 6166: if an estate's tax payment time was extended under section 6166, and (2) the estate treats as a deductible administrative expense under section 2053 either (A) interest the estate pays on tax under section 2001 for which payment was extended under 6166, or (B) interest on state estate, succession, legacy, or inheritance tax paid during that extension period - then, on the petitioner's motion, the Tax Court can reopen the case just to update its decision to reflect the estate's right to that deduction, and can hold a further trial on just that claim if the Tax Court thinks it is needed. An order deciding a motion under this subsection can be reviewed like a regular Tax Court decision, but only on the specific things it decided.