Recent advances in technology have made low cost, low power wireless sensors a reality. Clock synchronization is an important service in any distributed system, including sensor network systems. Applications of clock synchronization in sensor networks include data integration in sensors, sensor reading fusion, TDMA medium access scheduling, and power mode energy saving. However, for a number of reasons, standard clock synchronization protocols are unsuitable for direct application in sensor networks. In this paper, we describe a probabilistic service for clock synchronization that is based on the Reference Broadcast Synchronization protocol. In particular, we use the higher precision of receiver-to-receiver synchronization, as described in Reference Broadcast Synchronization protocol. We extend this deterministic protocol to provide a probabilistic bound on the accuracy of the clock synchronization service, allowing for a tradeoff between accuracy and resource requirement. We derive expressions to convert service specifications (maximum clock synchronization error and confidence probability) to actual protocol parameters(minimum number of messages and synchronization overhead). We also extend this protocol for maintaining clock synchronization in a multi hop network.