"Radiometric dating methods (40Ar-39Ar) show that the melted rock at the bottom of the Chicxulub crater is 65.07 +/- 0.1 million years old (Swisher et al., 1992). This date is extremely close to two separate age estimates for the K-T boundary in the North American interior (65.00+/- 0.04 Ma; Gradstein et al. (1995:102)) (65.4 +/- 0.1 Ma; Obradovich (1993). "

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It was narrowed down empiracally to a span of 40,000 years in the most recent study. That's enough time for evolution alone to wipe out the dinosaurs. This new study is based on a "firestorm" theory, a theoretical model of some kind.