In an attempt to resolve the issue, scientists at the Berkeley Geochronology Center (BGC), the University of California, Berkeley, and universities in the Netherlands and the United Kingdom have now determined the most precise dates yet for the dinosaur extinction 66 million years ago and for the well-known impact that occurred around the same time.
The dates are so close, the researchers say, that they now believe the comet or asteroid, if not wholly responsible for the global extinction, at least dealt the dinosaurs their death blow.
The revised dates clear up lingering confusion over whether the impact actually occurred before or after the extinction, which was characterized by the almost overnight disappearance from the fossil record of land-based dinosaurs and many ocean creatures. The new date for the impact -- 66,038,000 years ago -- is the same within error limits as the date of the extinction, said Renne, making the events simultaneous.

http://www.sciencedaily.com/releases/2013/02/130207141444.htm