I’m going to simplify the calendar even more; when the humans arrived they just named the months after the ones they were familiar with. The Game Start was thus on 12th February, Year 13 AF (After Flood).
I have also given Caribdus a moon, equal in size to Earth’s moon. The moon has a 28 day cycle; the 28th of each month is the night of the full moon. This allows for tides.