Understanding core formation in meteorite parent bodies is critical for constraining the fundamental processes of protoplanet accretion and differentiation within the solar protoplanetary disk. We report variations of 5 to 20 parts per million in W-182, resulting from the decay of now-extinct Hf-182, among five magmatic iron meteorite groups. These W-182 variations indicate that core formation occurred over an interval of similar to 1 million years and may have involved an early segregation of Fe-FeS and a later segregation of Fe melts. Despite this protracted interval of core formation, the iron meteorite parent bodies probably accreted concurrently similar to 0.1 to 0.3 million years after the formation of Ca-Al-rich inclusions. Variations in volatile contents among these bodies, therefore, did not result from accretion at different times from an incompletely condensed solar nebula but must reflect local processes within the nebula.