In terms of observable time, there exists a period that went unrecorded by the Xei. Having the prospect of living forever on their world, there was no need or inclination to mark down the earliest years of Civilization on Dyjian. They had no concept of their mortality, or a foreseeable end to their society.
Then, at what is estimated to be 4.3 thousand years later, the foundations of the world came unhinged and Dyjian had its first expansion. At that point the Xeigons decided to mark that day as the beginning of the First Epoch.
The Epochs are measured counting up from the end of one expansion to the beginning of the next; and from the beginning of the First Epoch to the start of the Second, 6.8 billion years passed.
There is a hypothesis among the Xei that Dyjian is a new, relatively virgin planet. Because in the grand total of their time on that world, it has expanded twice. Rather, thrice – but the Third Epoch is inconsistent with the previous two. Hence the theory that the planet is new.
However the actual age of Dyjian remains unknown. It's one of those ungodly numbers that, even if followed by a strict set of zeros, would mean nothing if not very little to the minds that received it. The literal age of the planet does not matter, but the Epochs do.
Time on Dyjian, as measured by the Eras, is more of a testament of survival and social change alongside planetary growth. The significance of the planet's growth leans more towards the establishment of nations and large Kyisaars, or Kyusoakin Communes, that spring up in newly formed, virgin lands (or abysmal depths, when considering the Ocean-bound Kyusoakin).
An example of the migration to virgin lands post-expansion is the exodus of Lucein and his people, and the colonization of the nine islands surrounding Vandanei.
Dyjian's Calendar.