A systematic way to introduce fault-tolerant and time-dependent properties into a complex software system is presented. The approach is based on an extended Petri Net model, called the G-Net, enhanced with a deterministic timing scheme. In the Timed G-Net (TGN) model, exception handling and timing mechanisms are used to introduce fault-tolerant and real-time properties. The basic idea is to use basic building blocks with timing properties to allow objects to treat a timing error by raising an exception and triggering some corrective actions. Using TGNs as these basic building blocks, we fur titer refine the model into a Smart Object (SMO) model. In each SMO, a TGN specification associated with a set of attributes, monitor/error-handler links and protocols support the construction of more reliable applications through an automatic transformer. This framework can contribute to alleviating the burden on the developers in designing a complex real-time dependable system. We use examples to show that a simplified real-time distributed air traffic control system can be modelled effectively using the SMO model.