Time synchronization plays an important role in the performance of wireless sensor networks. It can enhance the throughput and the lifetime of the network by improving the energy-efficiency, the freshness of collected data and reducing the network traffic and message conflicts. Due to the constraints on sensor nodes’ energy resources and the vulnerability of the distributed infrastructure of wireless networks, an efficient, scalable and accurate time synchronization protocol is desirable. This paper presents an accurate reactive protocol, named HRTS, that synchronizes the sensor nodes’ clock based on the node’s demand. It minimizes the synchronization region dynamically to the set of nodes which request synchronization. HRTS improves the accuracy of the synchronization procedure by measuring time parameters accurately and removing delays accordingly. Compared with the conventional time synchronization protocols like PCTS and TPSN, HRTS also reduces energy consumption by decreasing the traffic overheads especially when the network is not very dense.