In this paper we present a new model to formally represent complex information systems. This new formalism is very suitable to specify time requirements (both time consumption due to the performance of tasks and timeouts) as well as to represent data communication among different components of a system. In addition to provide the theoretical framework, we show how this formalism, based on finite state machines, has been used as the basis to develop a real information system. This exercise convinced us that a formal approach to develop complex systems can facilitate some of the development phases. In particular, the testing phase, more precisely, how to choose which tests to apply, is simplified since tests are automatically extracted from the specification.