Time based TDOA localization systems require time or clock synch-ronization between receivers such as cellular base stations, satellites, and sensor nodes. Imperfection of time synchronization causes degradation in positioning accuracy. However if we know about its characteristics and how to estimate the clock offsets, the localization system can be properly calibrated to provide good quality of services. In this paper, hence, we present how to derive a localization error vector with independent clock offset, and illustrate its effect on the positioning errors, and then, provide a simple method of TDOA clock offset estimation from the observation of error vectors.