Adaptation is an important requirement for mobile applications due to the varying levels of resource availability that characterises mobile environments. However, without proper control, multiple applications can each adapt independently in response to a range of different adaptive stimuli, causing conflicts or suboptimal performance. In this paper, we present a policy-driven approach for mobile adaptive systems that can overcome the aforementioned problems. Our system is based on a policy language derived from the event calculus logic programming formalism. Important characteristics of our policy language are its support for explicit time-dependency expressions and its simple and user-friendly syntax.