• 1 Laboratoire d'Electrotechnique, de Télécommunication et d'Informatique Appliquée
• 2 Institut de Recherche en Communications et en Cybernétique de Nantes

For the formal verification of the concurrent or communicating dynamic systems modeled with Petri nets, the method of the unfolding is used to cope with the well-known problem of the state explosion. An extension of the method to the non safe time Petri nets is presented. The obtained unfolding is simply a prefix of that from the underlying ordinary Petri net to the time Petri net. For a certain class of time Petri nets, a finite prefix capturing the state space and the timed language ensues from the calculation of a finite set of finite processes with valid timings. The quantitative temporal constraints associated with these processes can serve to validate more effectively the temporal specifications of a hard real-time system.