Time synchronization is an essential problem for WSNs. The synchronization performance is mainly affected by two factors: the uncertainty in the message delay, and the clock drift. Most solutions on time synchronization in WSNs have concentrated on one of the two factors, and neglected the other. In this paper, we consider both of them and propose a novel time synchronization protocol for WSNs. In the protocol, we model the variation of the clock drift and develop algorithms to estimate and compensate the drift. Meanwhile, the clock offset is corrected continuously avoiding the time discontinuity which is a prevalent drawback of existing synchronization protocols. Additionally, we extend the bound-drift clock model by considering both the absolute drift and the relative drift. Mathematical analyses and simulations are presented in this paper, and verify the effectiveness of our synchronization protocol.