—This paper describes the Secure Time Synchronization (STS) protocol that enables client and server mutual authen-tication, supports the property of non-repudiation, and offloads the negotiation and authorization phases to an Authorization Server (AS). We also propose a solution for bootstrapping time synchronization to solve the problem of certificate validation that depends on time. We analyze the main security properties of STS with the ProVerif tool, implement STS by extending OpenNTPD, and compare its precision to unauthenticated NTP.