The Chatter, first of the Eras and embodiment of Prehistory, was born when Humanity became Humanity in a meaningful, messy, metaphysical way that we cannot quite define.
The timeline set by Destiny, who declared that all things shall stem from this one set of events that shall not change under any circumstances whatsoever.
The first way of counting years, when all was undoubtedly chaos and bad (mostly). Contains everything from the Before until the inciting incident. You know what that was.
The Chatter, first of the Eras and embodiment of Prehistory, was born when Humanity became Humanity in a meaningful, messy, metaphysical way that we cannot quite define.
The Songbird, second of the Eras and embodiment of Antiquity, was born when Humanity first began to Create meaningfully creative things, which can be described in many ways that I will not describe here.
The second way of counting years, up from year 0. When the world started properly progressing.
The Battle of Hastings marks the point at which Layer-Gamma diverges from Layer-Alpha. Unlike Layer-Beta, which diverged at an unknown point, the Battle of Hastings is undoubtedly the inciting incident for the divergence of Layer-Gamma.