This paper summarizes an actor-based middleware in Java for the development of time-dependent distributed systems. The approach centres on the separation of concerns. Functional, synchronization, control and configuration aspects of an application can separately be dealt with in a design so as to favour the fulfilment of the application requirements. Actors are encapsulation units of data and behaviour and are modelled as finite state machines. They enable ubiquitous computing. Actors have location transparent names and can migrate, possibly autonomously, from node to node of an heterogeneous computing infrastructure like Internet, for accomplishing their computational task. Actors are based on an explicit messaging system. Crosscutting concerns in group of actors can be programmed in special actors (synchronizers) which transparently filter message exchanges and superimpose to them timing and synchronization constraints which directly affect message scheduling. The paper describes the implementation of a real-time multimedia QoS synchronizer.