We consider a time-based model for quota trading in mobile data networks. In particular we present a dynamic program to characterize the behavior of mobile users when they have the option to trade data with other users during the month. The user will decide how much data to consume at each time period based on the utility that can be gained, the amount of quota remaining and the price available for trading. In contrast to past work on quota trading, our model explicitly takes into account the time remaining in the billing period when users make trading decisions. In addition, we utilize a VCG-based trading strategy to ensure that users truthfully reveal the value they assign to traded data. We present two variants of the model that differ based on how much foresight users are assumed to have with respect to future prices. We use our model of quota dynamics to estimate the gain in user utility when quota trading is introduced. This in turn translates into an increased price that the operator can charge for each level of quota. We also compare the utility benefits of quota trading with an alternative scheme in which the users buy and sell additional data from the operator rather than the other users.