In this paper, we handle the problem of conformance testing for data-flow critical systems with time constraints. We present a formal model (Variable Driven Timed Automata) adapted for such systems inspired from timed automata using variables as inputs and outputs, and clocks. In this model, we consider urgency and the possibility to fire several transitions instantaneously. We present a conformance relation for this model and we propose a test generation method using a test purpose approach, based on a region graph transformation of the specification.