In the world of Andrzej Sapkowski's Witcher books, how many days long is a year? How many days long is a synodic month (the average time between two full moons)?

I used to assume that the stories take place on an alternate Earth with the same astronomy as our Earth. That is a convenient default for fantasy stories, and is supported by the books using our month names. However, The Tower of the Swallow (Wieża Jaskółki) chapter 1 seems to contradict this, because it states that the moon is always waning on the autumn equinox. On Earth, the moon phases at the same date of the year keep changing quickly enough, even short-lived humans would notice that they differ within a decade.

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    Earth's year isn't even close to an integer multiple of a lunation -- 29 1/2 days doesn't go into 365 1/4 without a significant remainder. Therefore the lunar phase on any given equinox/solstice will be significantly different from last year, never mind over a decade. – Zeiss Ikon Dec 31 '19 at 19:39
  • @ZeissIkon Sure, but the moon can be waning at the same date in several consecutive years. – b_jonas Dec 31 '19 at 19:40
  • Not really. It changes by close to 9 days per year -- that is, the phase on autumn equinox will be roundly nine days earlier than on the previous one. Might be waning two years in a row, never more than that. – Zeiss Ikon Dec 31 '19 at 19:42

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