Heaven's Contention was the genuine starting point for civilization and history in the world. It was a war of the Gods, all attempting to defeat the ambitious rivals that would rule over this new land, so that they may do so for themselves. Heaven's Contention began shortly after The Enlightenment, an event and short period of time when the Gods appeared in the world from The Void, sharing their knowledge and power, converting the native inhabitants that would serve them into like-beings. The tireless battle between the divines raged for 10 years straight without rest on the Godly Stage.
It unarguably ended when the last two remaining participants, Ryart the Gold Dragon and the demonic Zera, killed each other in their final battle. Zera, able to stave off death, was its victor, though the wounds he suffered from the dragon were fatal even for a god. With the gods leaving their stage, in a sense, mortals began fending for themselves, making the world theirs again. This was a period of consolidation and conquering for some, leading to great empires such as Sigrogana's and Oniga's.
This was not the end of the Gods, however, as some abstained from fighting and hid themselves away, such as the patron deity of the Glykin. If they still exist to the present day, no one is sure, as if so, they keep to themselves.
Year 0 is the end of Heaven's Contention, and that ended when Ryart and Zera killed each other. The world existed before that, though, for an undetermined amount of time because no one was counting before then.