One important aspect in many applications of wireless sensor networks (WSNs) is time synchronisation. There are still some problems to be solved for time synchronisation, such as error and imbalance of time synchronisation precision, causing the fluctuations among nodes' time and being even worse for those with a larger number of hops, producing unstable communication links for the network. In this paper, we introduce an extensible time synchronisation protocol (ETSP) for WSNs and nodes get synchronised through time stamps from the root node directly or indirectly. We improve the least square method for linear regression to get better performance of time drift and spread the time information from the root node to the whole network. The proposed method reduces fluctuations in terms of time among nodes especially for the nodes far from the root, enhancing stability in large-scale networks. Simulation and experimental results show good performance of our method.