The Zambaso Calendar was formed a couple of hundred years after The Great Exodus but still sets that day as it's point zero.
The Great Exodus marked a new era for the Zambasonian people when the gods decided to leave the earth, and although no one was awake to witness it, at the dead of night, it is said that a great light shone down upon the Deenja island. The Great Exodus is, thus, also called the Illuminated Night.
The calendar is based around the moon, Kielon as it is known in the east, and every new moon a new month starts, as it was calculated that the IN happened during a new moon, which also fits with old descriptions detailing a "moonless night with no stars on the horizon" although the sources are questionable.