The concept of real time has different meanings in the systems and theory communities. Thus, the existing formal real-time models do not capture all the practically relevant aspects of such computations. This article proposes a new definition that, we believe, allows a unified treatment of all practically meaningful variants of realtime computations. We use the developed formalism to mode two important features of real-time algorithms, namely the presence of deadlines and the real-time arrival of input data. We also emphasize the expressive power of our model by using it to formalize aspects from the areas of real-time database systems and ad hoc networks. We offer formulations of the recognition problem for real-time database systems and of the routing problem in ad hoc networks. Finally, we suggest a variant of our formalism that is suited for modelling parallel distributed real-time algorithms. We believe that the proposed formalism is a first step towards a unified and realistic complexity theory for real-time parallel computations.