Upon the expiration of the time allowed for filing a notice of appeal, if no such notice has been duly filed within such time; or
If the Supreme Court directs that the decision of the Tax Court be modified or reversed, the decision of the Tax Court rendered in accordance with the mandate of the Supreme Court shall become final upon the expiration of 30 days from the time it was rendered, unless within such 30 days either the Secretary or the taxpayer has instituted proceedings to have such decision corrected to accord with the mandate, in which event the decision of the Tax Court shall become final when so corrected.
Notwithstanding subsection (a), if, within 1 year after the date the decision of the Tax Court becomes final under subsection (a) in a case to which this subsection applies, the taxpayer files a motion in the Tax Court for a redetermination of the amount of interest involved, then the Tax Court may reopen the case solely to determine whether the taxpayer has made an overpayment of such interest or the Secretary has made an underpayment of such interest and the amount thereof.
If the Tax Court determines under this subsection that the taxpayer has made an overpayment of interest or that the Secretary has made an underpayment of interest, then that determination shall be treated under section 6512(b)(1) as a determination of an overpayment of tax. An order of the Tax Court redetermining interest, when entered upon the records of the court, shall be reviewable in the same manner as a decision of the Tax Court.