Perfect failure detectors can correctly decide whether a computer is crashed. However it is impossible to implement a perfect failure detector in purely asynchronous systems. We show how to enforce perfect failure detection in timed distributed systems with hardware watchdogs. The two main system model assumptions are: each computer can measure time intervals with a known maximum error; and each computer has a watchdog that crashes the computer unless the watchdog is periodically updated. We have implemented a system that satisfies both assumptions using a combination of off-the-shelf software and hardware.