The traditional time given for the building of the Great Pyramid is the 4th Dynasty, during the reign of Pharaoh Cheops or Khufu. The date range is anywhere from 2500-2100 BCE.

Zahi Hawass, Egypt's Antiquities Director is the leading proponent of the traditional date.  Hawass obviously wants his Egyptian forefathers to get credit for the glory of the pyramids, and not some alien visitors from a previous time. The archaeological explorations and digs by Hawass at Giza have uncovered the barracks of the builders and other aspects of their culture such as the bread, fish and beer they feasted upon.

There is thus scientific and political muscle to back the traditional date of the Pyramid's construction.

A small but growing clique, however, argues for a much older date for the erection of the Pyramid and the Sphinx. The major motivation for this group seems to be the justifying of a time frame that could include the visitation to earth of intelligent aliens.  Because the Pyramid's building lines and measures display knowledge that was thought to have been discovered only in modern times, such as the value of pi, the circumference of the earth, the speed of light (Julian Gray insists the Pyramid displays this!), and Fibonacci series, to name a few, this group wants to attribute the Pyramid's prescience not to Egyptians in historic times, but to aliens.