Time synchronization is necessary to a Wireless Sensor Network (WSN). Existing time synchronization methods are mostly designed for large scale ad hoc network and are not suitable for small star network like Wireless Physiological Information Sensor Network (WPISN). In this article we present a simple and power-efficient time synchronization method designed specially for WPISN, Reference on Demand Time Synchronization method (RDTS), which employs the Near-Field Magnetic Communication (NFMC) technology. NFMC offers significant power, cost, and size advantages over RF communication.