The Moon Phases calculation is off. More on moon phases under (*). I'm at 16 Hammer 1485, moon is waning (!) gibbeous. Go forward 5 days in one adjustment, to 21 Hammer 1485, and moon is full. From waxing gibbeous to full in 5 days, yes. From full to waning gibbeous in 5 days, yes. From waning gibbeous to full in 5 days ... not unless the moon cycles through phases in under 5 days :). Moon phases for reference, in order: Waxing Crescent First Quarter Waxing Gibbous Full Moon Waning Gibbous Last Quarter Waning Crescent New Moon Love the script! Thanks so much for putting this together! (*) And Faerun moon nerdery from the wiki. Looks like it's close to earth-like, but with a slightly more "orderly" progression of the phase in relation to the orbit of its parent. This actually makes for easier calculations. I don't know how or whether the spellplague affected the phases of the moon at all. Selûne was full at midnight on Hammer 1, 1372 DR , and subsequently every 30 days, 10 hours, and 30 minutes. Selûne was also full at midnight on the first day of every leap year. Selûne made exactly 48 synodic revolutions every four years, so the phase of Selûne was the same at the same time on the same date every four years. Selûne was generally full around the first day of each month, give or take a day; festival days served to keep the moon's phases consistent across calendar months and years.