Believed by mainstream Egyptologists to have been constructed in approximately 20 years and 20 days, the generally accepted estimated date of its completion is c. This date is loosely supported by archaeological findings which have yet to reveal a civilization (of sufficient population size or technical ability) older than the fourth dynasty in the area.
The Great Pyramid is the oldest and largest of the three pyramids in the Giza Necropolis bordering what is now Cairo, Egypt in Africa.
A ' Khufu' cartouche is visible on the southern roof blocks of ' Campbell's chamber', the topmost one and a ' Khnum-Khuf' cartouche is found on the south wall of ' Lady Arbuthnot's' chamber.
Here's a diagram of the hieroglyphs: Khufu is believed to have reigned from around 2589 to 2566 BC as the second pharaoh of the Fourth Dynasty.
No 'forger' could possibly have reached in there after the blocks had been set in place - blocks, I should add, that weigh tens of tons each and that are immovably interlinked with one another.