It was proved that Euler-Maxwell systems converge globally-in-time to drift-diffusion systems in a slow time scaling, as relaxation times go to zero. The convergence was established to the Cauchy problem with smooth initial data being sufficiently close to constant equilibrium states. In this talk, we establish error estimates between periodic smooth solutions of Euler-Maxwell systems and those of drift-diffusion systems. We also establish similar error estimates for Euler-Poisson systems in place of Euler-Maxwell systems. The proof of these results uses stream function techniques together with energy estimates. This is a joint work with Yue-Jun Peng and Liang Zhao.