A new software tool for hard real-time system timing constraint modelling and validation is presented. The tool consists of a set of predefined timed coloured Petri net (TCPN) structures. The structures include built-in mechanisms, which detect missing timing constraints and make it possible to validate the timing correctness of the modelled system. The final model of the system is a hierarchical TCPN following the execution and simulation rules of CPN/Design software. The paper focuses on presenting the construction of the software tool and the guidelines for applying it in real-time system development.