31 C.F.R. § 306.35
The interest on Treasury securities accrues and is payable on a semiannual basis unless otherwise provided in the circular offering them for sale or exchange. If the period of accrual is an exact 6 months, the interest accrual is an exact one-half year's interest without regard to the number of days in the period. If the period of accrual is less than an exact 6 months, the accrued interest is computed by determining the daily rate of accrual on the basis of the exact number of days in the full interest period and multiplying the daily rate by the exact number of days in the fractional period for which interest has actually accrued. A full interest period does not include the day as of which securities were issued or the day on which the last preceding interest became due, but does include the day on which the next succeeding interest payment is due. A fractional part of an interest period does not include the day as of which the securities were issued or the day on which the last preceding interest payment became due, but does include the day as of which the transaction terminating the accrual of interest is effected. The 29th of February in a leap year is included whenever it falls within either a full interest period or a fractional part thereof.
31 C.F.R. §306.35