We discuss receiver clock correction and associated performance bounds for passive emitter localization using TDOA measurements from asynchronous sensor networks. In the considered system, passive receiving sensors are augmented with beacons at known locations that perform approximately periodical transmissions, used for calibration of the system synchronization. The precise transmission times as well as the transmission interval of the beacon messages are unknown. Similarly the transmissions of the target are irregular and do not, in general, occur simultaneous with the beacon transmissions. The clocks of each sensor are described by a linear error model. Based on that, we compare different snapshot based approaches for clock correction and derive theoretical limits for the localization of the target based on the modified Cramér-Rao lower bound (MCRLB). Simulation results are presented to illustrate the theoretical findings and show that our proposed estimators perform close to the MCRLB. Additional experimental results verify the analysis and the approach in a realistic large scale scenario.