The Ibbotson U.S. 30 Day Treasury Bill Index is an unweighted index which measures the performance of one-month maturity U.S. Treasury Bills. Each month a one-bill portfolio containing the shortest-term bill having not less than one month to maturity is constructed. To measure holding period returns for the one-bill portfolio, the bill is priced as of the last trading day of the previous month-end and as of the last trading day of the current month.