This paper presents an improved method for guaranteed state estimation of discrete-time linear-time varying systems affected by disturbances, noises and structured uncertainties modeled as interval uncertainties. Under the hypothesis that the disturbances and the noises are bounded, a zonotopic outer approximation of the state estimation domain is computed offering good performance and low complexity compared to the existing methods. The size of this zonotope is decreased by solving an off-line optimization problem. The advantages of the proposed approach are illustrated via a numerical example.