The Gemarah on 7a seems to suggest that the day of Yom Kippur is Mchaper one's sins once Amud Hashachar comes. But the Gemara then says that if you die at Amud Hashacar or during the day while choking you weren't mecahper? Why not? If you commit an aveirah it should be mechaper at that second.
Eitan Selter, Monsey, NY, USA
The person died instantly. Rashi in Shevuos (13b, DH v'Chanaktei) writes that not a single moment of the day of Yom Kippur passed after the transgression. The Ritva there adds that Yom Kippur does not atone for a person who died in the middle of the Aveirah, since he was not alive for any part of Yom Kipper after the Aveirah.