In today's wireless data market, the growing data demand and more severe market competition force mobile network operators (MNOs) to offer more flexible wireless data services. Many MNOs are experimenting with novel data mechanisms generalizing the traditional three-part tariff data plan involving a fixed data cap. The generalization is often in terms of providing additional flexibility in terms of time, user, and location. The rollover mechanism, allowing the unused data in the current month to be consumed in a future month, is a time-flexible data mechanism. As an illustrative example, we focus on studying the economics and optimizations of wireless data plans with time flexibility. We consider a framework that incorporates various time-flexible data plans (as well as the non-flexible benchmark). We characterize the degree of flexibility of different plans, and investigate the economic impact of time flexibility on both users' payoffs and MNOs' profits. We incorporate various factors in our study, such as the adoption of multiple data caps and market competition. We also discuss how time flexibility can interact with user flexibility. Finally, we outline some open problems that deserve future work from the community.