HMAC is a MAC protocol developed for wireless sensor networks at LTU and is based on time division. Consequently it is highly susceptible to time deviations between nodes, but is good in terms of low power consumption. There is currently no time synchronization implementation for the HMAC protocol, which has an impact on the reliability and performance of the protocol. In this thesis we focus on implementing time synchronization for HMAC on TinyOS, ensuring reliable function of HMAC, therefore enabling its use in production environments. Clock drift was measured on our target platforms and the performance of the reference flooding time synchronization protocol (FTSP) implementation was evaluated. We optimized the performance of FTSP on our platforms by adding microsecond precision support and fixing FTSP-related bugs present in the radio driver layer. The implementation of HMAC for TinyOS was redesigned, with the goal of improving performance, reliability and code manageability. We also integrated FTSP directly into HMAC, improving HMAC reliability even further. The final tests in this thesis show the achieved reliability, synchronization and low power consumption.