Zadoff-Chu (ZC) sequences have been proposed for time synchronization of OFDM systems due to their ideal correlation properties. In this paper, we examine the performance of ZC sequence based time synchronization in the presence of large carrier frequency offsets (CFOs). We show that the performance of synchronization in the presence of integer frequency offsets (IFOs), is dependent on the root index of the ZC sequence and some sequences may fail entirely in the presence of IFOs. We develop the conditions to choose such root indices for which inter symbol interference can be avoided for a given maximum IFO. We then design a preamble based on the selected ZC sequences to perform timing synchronization as well as IFO estimation. We also propose a scheme to further enhance the performance of time synchronization through the estimated IFOs. The performance of the proposed algorithms is evaluated through simulation results.