Most successful efforts to determine the ages of impact events are based on the isotope geochronology of crystalline or glassy impact melts. Studies of impact sites on Earth show that many form without significant melt production, meaning that traditional geochronologic approaches can yield unsatisfying results. We describe here an alternative approach based on theoretical calculations that even brief thermal events related to impact can reset the isotopic systematics of unmelted target rocks. Thermochronometers based on the production of radiogenic 4He in accessory minerals are particularly amenable to complete resetting by impact and should yield robust impact ages if helium systematics are not further disturbed during post-impact thermal events. We illustrate the utility of this method through a presentation of a new zircon (U-Th)/He date for the Haughton impact structure, Canada, of 23.5 ± 2.0 Ma.