For an autonomous physical agent, such as a moving robot or a person with their mobile device, performing a task in a spatio-temporal environment often requires interaction with other agents. In this paper we study adhoc collaborative planning between these autonomous peers. We introduce the notion of decentralized time geography, which differs from the traditional time-geographic framework by taking into account limited local knowledge. This allows agents to perform a space-time analysis within a time-geographic framework that represents local knowledge in a distributed environment as required for ad-hoc coordinated action between agents in physical space. More specifically, we investigate the impact of general agent movement, replacement seeking, and location and goal-directed behavior of the initiating agent on the outcome of the collaborative planning. Empirical tests in a multi-agent simulation framework provide both a proof of concept and specific results for different combinatio...