When tuning operating system and network code, profiling programs, analyzing message interarrival times, and accurately measuring device characteristics, a high resolution clock is often indispensable, as one cannot measure service time distributions without one. This note describes a microsecond clock that we designed and built for Sun 3 and Sun 4 workstations¹. One can measure average service times without a high resolution clock. This paper explains how to measure average times with high precision in the absence of such a clock. We pose and answer the question: "how many measurements are needed to report timing data to three significant digits?"