Predicts the accumulated H-period return at time t. Note that when t >= T-H+1, we are
predicting the future accumulated returns, as the next H-period ends at T-H+1-H = T+1,
which extends beyond the currently observed data.
t - a time
the accumulated H-period return predicted using the information at time t
Gets the difference between predicted accumulated returns and realized accumulated
returns at time T-H+1, the last historical fully realized accumulated return.
Starting from time T-H+2, the actual realized returns are not known yet as some
returns are in the future.