Many Phanerozoic time-scale boundaries are characterized by oceanic anoxia and mass extinction events with the deposition of black shale. The Re-Os isotope system in black shale can be used to provide depositional ages for these rocks, thus yielding direct radiometric ages for time-scale boundaries. We demonstrate that the Re-Os black shale geochronometer can yield precise ages useful for time-scale research through study of the Devonian-Mississippian boundary within the Exshaw Formation, Canada. The Re-Os date determined places the Devonian-Mississippian boundary at 361.3 ± 2.4 Ma (2σ including λ uncertainty, model 1, mean square of weighted deviates = 1.2), in accord with recent calibration interpolated from U-Pb zircon dates (360.7 ± 0.7 Ma) for the Hasselbachtal section, Germany.