97,445 BCE

The date 100,000 B.C.E. is the approximate time when The Forerunners fired the Halos, what the Covenant refer to as The Great Journey. The precise activation date was during 97,448 B.C.E., and it was meant to end the Forerunner-Flood War. The activation killed all sentient life in three radii of the galactic center including, it is believed, the Forerunners themselves. The date, about 100,000 years ago, is around the time our Homo Sapien ancestors began migrating out of Africa. It is when the Halo 3: The Cradle of Life comics take place.

Mendicant Bias
The Forerunner AI known as Mendicant Bias was designed to battle the flood but he was seduced by Gravemind and descended into rampancy. He disrupted the plans to retreat to the Maginot Spheres, or Shield Worlds, forcing the forerunner to fire the rings and subsequently commit "mass suicide". He later repented to Spartan-117 (Master Chief) through the terminals on the levels, promising to atone for his past actions by helping the Master Chief fire Halo and help him escape (how he does this is unknown).