Detection and identification of failures is a critical task in the automatic control of large and complex systems. In the realm of discrete event systems, Sampath et al. proposed a new approach to failure diagnosis that models the logical behavior of the considered system in terms of state machines and produces an extended observer called a diagnoser for computing diagnoses. We extend this approach to the diagnosis of timed discrete event systems whose temporal and logical behavior are modeled by a framework proposed by Brandin and Wonham. We use a simple real-world factory conveyor example to demonstrate our modeling and diagnosis approach.