For Wireless Sensor Networks (WSNs), utilizing the same set of measurement data for simultaneous localization and time synchronization is potentially useful for achieving higher estimation accuracy, lower communication overhead and power consumption. However, localization and time synchronization are traditionally treated as two separate problems. In this paper, we propose to use Semidefinite Programming (SDP) to jointly solve the two problems at the same time. Two scenarios, Sensor Nodes (SNs) with sufficient and insufficient number of neighboring beacons, are considered. The Cramer-Rao Lower Bound (CRLB) is then derived. Simulation results are presented to contrast the performance of the proposed Simultaneous Localization and Time Synchronization-SDP (SLTS-SDP) and Generalized SLTS-SDP (GSLTS-SDP) algorithms with the conventional SDP method and the corresponding CRLBs.