Dynamic Spectrum Sharing (DSS) has a great potential in fully utilizing the scarce spectrum resources, and heterogeneous two-tier network has been regarded as one major solution for achieving it. Without privacy protection in operation-time, however, the primary users will be reluctant to share their spectrum with secondary users. In this paper, we present PriDSS in two-tier wireless networks, the first scheme for the administrator of a dynamic spectrum sharing system to select secondary users in a differentially operation-time private manner. First, we describe the operation-time inference attacks on the traditional secondary users auction without privacy. Then, we bring up a ranking metric to quantify the administrator's preference for secondary users. Moreover, based on the exponential mechanism, we calculate the probability of each secondary user being selected as a winner through the ranking metric. Finally, a truthful payment method is designed according to that probability. Extensively theoretical analysis and evaluations show that PriDSS can simultaneously achieve truthfulness, approximate social welfare maximization, and differential operation-time privacy.