We introduce a time-limited neighbor detector service for mobile ad hoc networks, which enables a mobile device to detect other nearby devices in the past, present and up to some bounded time interval in the future. Our motivation lies in the emergence of a new trend of mobile applications known as proximity-based mobile applications, which enable a user to communicate with other users within some defined range and for a certain amount of time. Neighbor discovery is a fundamental requirement for these applications and is not restricted to the current neighbors but can include past or future neighbors. To implement the time-limited neighbor detector service, we apply an approach based on virtual mobile nodes. A virtual mobile node is an abstraction that is akin to a mobile node that travels in the network in a predefined trajectory. In practice it can be implemented by a set of mobile nodes based on a replicated state machine approach. In this paper, we assume that each node can accurately predict its own locations up to some bounded time interval in the future. Thus, we present a time-limited neighbor detector algorithm that uses a virtual mobile node that continuously travels in the network, collects the predicted locations of all nodes, performs the neighborhood matching between nodes and sends the list of neighbors to each node. We show that our algorithm correctly implements the time-limited neighbor detector service under a set of conditions.