31 C.F.R. § 359.15
The most recently announced composite rate applies to a bond during its next semiannual rate period. A bond's semiannual rate periods are consecutive six-month periods, the first of which begins with the bond's issue date. This means that there can be a delay of several months from the time of a composite rate announcement to the time that rate determines interest earnings for a bond. For example, if you purchased a bond in April, its semiannual rate periods begin every April and October. At the beginning of the semiannual rate period in April, the most recently announced composite rate would have been the rate we announced the previous November. This rate will determine interest earnings for your bond for the next six months, through the end of September. At the beginning of the semiannual rate period in October, the most recently announced composite rate would be the rate announced the previous May. This rate will determine interest earnings for your bond through the end of the following March. However, if you purchased a bond instead in May, its semiannual rate periods begin in May and November. Therefore, the composite rates announced in May and November will apply immediately to this bond. (See appendix C to part 359 at § 2 for a discussion of rate lag.)
31 C.F.R. §359.15