base period T-bill rate
(4) Base period T-bill rate For purposes of this subsection, the term “base period T-bill rate” means the annual rate of interest determined by the Secretary to be equivalent to the average of the 1-year constant maturity Treasury yields, as published by the Board of Governors of the Federal Reserve System, for the 1-year period ending on September 30 of the calendar year ending with (or of the most recent calendar year ending before) the close of the taxable year of the shareholder.