The time-­‐dependence of the Bragg diffraction by one-­‐dimensional photonic crystals and its influence on the short pulse reflection are studied in the framework of the coupled-­‐ wave theory. The indicial response of the photonic crystal is calculated and it appears that it presents a time-­‐delay effect with a transient time conditioned by the extinction length. A numerical simulation is presented for a Bragg mirror in the x-­‐ray domain and a pulse envelope modelled by a sine-­‐squared shape. The potential consequences of the time-­‐delay effect in time-­‐dependent optics of short-­‐pulses are emphasized .