In this paper, we consider the problem of routing, modulation, and spectrum assignment (RMSA) for immediate and advance reservation requests in elastic optical networks. We design a two-dimensional resource model for maintaining resource state information in both spectrum and time domains. For the purpose of minimizing the blocking probability and increasing spectrum utilization, we develop a two phase RMSA algorithm that attempts to decrease spectrum resource fragmentation by scheduling the requests in a manner that takes into account the holding time of each request. We design a simulation to evaluate the performance of the proposed algorithm, and the experiment results show that our proposed two phase RMSA algorithm can greatly reduce blocking probability and obtain high spectrum utilization.