  This letter investigates a wireless communication system deploying
distributed reconfigurable intelligent surfaces (RISs). Existing works have
assumed that perfect timing synchronization is available among all the
cooperative RISs. For practical considerations, we first study cooperative
reflection design for multi-RIS-aided communication systems taking into account
timing synchronization errors. We aim to minimize the mean-squared error of the
recovered data in the presence of timing offsets subject to the unit modulus
constraints imposed on the phase shifts of the RISs. In order to handle this
sophisticated nonconvex problem, we develop a computationally-efficient
algorithm based on the majorization-minimization framework where the RIS phase
shift matrices and the timing offset equalizer are respectively developed in a
semi-closed form and a closed form. Numerical examples validate the improved
performance of our proposed design compared with various schemes.
