Why does the era of the books of Recluse not start, for example, with the arrival on the island or with the birth of the founder?

The dates in the chronology begin with the year that the rationalist colonists landed. Those dates show up in Magi’i of Cyador and Scion of Cyador. The term A.F., refers to After the Fall. Other cultures around the world may use other dates, but I used the founding of Cyad as the base date.