JBCrowson
Maester
One of the other things that makes the calendar dates of full moons vary in RW is the fact that calendar months vary in length from 28 to 31. That is something you could easily reproduce in your story world.
I also have to opine readers are not going to be whipping out their calculators while reading your book and saying "Mmmm, that full moon was a whole two days early! If a full moon fell on the equinox last year, I find it disturbingly implausible that there was also one on the solstice this year. I refuse to read more of this calendrically challenged nonsense."
So maybe don't sweat it?
I also have to opine readers are not going to be whipping out their calculators while reading your book and saying "Mmmm, that full moon was a whole two days early! If a full moon fell on the equinox last year, I find it disturbingly implausible that there was also one on the solstice this year. I refuse to read more of this calendrically challenged nonsense."
So maybe don't sweat it?

Myth Weaver