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Time synchronization is a crucial mechanism for wireless sensor networks, so it is a natural target of the adversaries. Errors in data aggregation and failures of other mechanisms will happen as a result of imprecise time synchronization. The well-known mu TESLA protocol can be employed to authenticate broadcast packets in secure time synchronization mechanisms. Unfortunately, nodes and base station should be loosely synchronized for mu TESLA to function. It appears to be a dilemma that time synchronization is required to achieve time synchronization. A separate synchronization mechanism is employed in some other scheme just to escape from such a dilemma. It is shown in this paper that it is NOT necessary to assist mu TESLA with any separate mechanism. Shaking off an unnecessary burden, we propose ASTS (agile secure time synchronization), a much more lightweight scheme. We build an experiment network with 32 mica2 motes to evaluate the performance of ASTS. It is shown that the synchronization error of ASTS is as low as several microseconds, matching the best possible time synchronization protocols.