I bonds earn interest from the first day of the month you buy them. Twice a year, we add all the interest the bond earned in the previous 6 months to the main (principal) value of the bond. That gives the bond a new value (old value + interest earned). Over the next 6 months, we apply the new interest rate to that entire new value. This is called semiannually compounding (adding value 2 times a year). That way, your money grows not just from the interest percentage but from the fact that the interest is calculated on a growing balance.