We determine this rate by compiling 6-month Treasury securities rates as of the close of business for each day of the previous three months and calculating the monthly average for each month, rounding each monthly average to the nearest one-hundredth of one percent. We then determine the short-term savings bond rate by taking 85 percent of the three-month average and rounding the result to the nearest one-hundredth of one percent. For bonds entitled to interest accruals at the short-term savings bond rate, that rate applies to the bond's first full semiannual interest accrual period following each announcement of the rate.
31 C.F.R. § 351.11