Exact Date Pinned to Great Pyramid's Construction?

According to new research by a group of Egyptian scientists the Great Pyramid of Giza's construction began on August 23, 2470AD. The researchers based it off of the appearances of the star Sothis. Every year during the Nile floods, the star Sothis would rise early in the sky after a long absence.This marked the beginning of an inundation period, and all building began at the beginning of the inundation period. The year is marked as so, because this is when King Khufu took power and was supposed to be buried in this tomb. The team believes the Egyptians observed the star from July 17 to 19, and the inundation period began 35 days later—on August 23.

http://news.nationalgeographic.com/news/2009/09/090921-great-pyramid-giza-date-built.html