We develop an analytical solution to the dynamic portfolio choice problem of an investor with power utility defined over wealth at a finite horizon, who faces a time-varying investment opportunity set, parameterized using a flexible vector autoregression. We apply this framework to study the horizon effects in the allocations of equity-only investors, who hold a mix of value and growth indices, and a more general investor, who also has access to Treasury bills and bonds. We find that the mean allocation of equity-only investors is heavily tilted towards value stocks at short-horizons, but the magnitude of this tilt declines dramatically with the investment horizon, implying that growth is less risky than value at long horizons. Investors with access to bills and bonds exhibit similar behavior, when value and growth tilts are computed relative to the total equity allocation of the portfolio. However, after accounting for the propensity of these investors to increase their total equity allocation as the horizon increases, the mean value tilt of the optimal allocation is shown to be positive and stable across time.