A portfolio manager uses a valuation model to estimate the value of a bond portfolio at USD 125.00 million. The term structure is flat. Using the same model, the portfolio manager estimates that the value of the portfolio would increase to USD 127.70 million if all interest rates fall by 20 bps and would decrease to USD 122.20 million if all interest rates rise by 20 bps. Using these estimates, which of the following is the effective duration of the bond portfolio closest to?