We present a tool-supported approach to the validation of system-level timing properties in formal models of distributed real-time embedded systems. Our aim is to provide system architects with rapid feedback on the timing characteristics of alternative designs in the often volatile early stages of the development cycle. The approach extends the Vienna Development Method (VDM++), a formal objectoriented modeling language with facilities for describing real-time applications deployed over a distributed infrastructure. A new facility is proposed for stating and checking validation conjectures (assertions concerning real-time properties) against traces derived from the execution of scenarios on VDM++ models. We define validation conjectures and outline their semantics. We describe the checking of conjectures against execution traces as a formallydefined extension of the existing VDM++ tool set, and show tools to visualise traces and validation conjecture violations. The approach and tool support are illustrated with a case study based on an in-car radio navigation system.