The excelent Astrology Article by Steve McDonnald indicates that the period of precession for Kethira is 21,000 years. Over 21,000 years the direction of the North Pole moves around in a large circle in the sky. In turn this causes the time of the equinoxes and solstices to move around the year.
So if that was the case over 1000 years the 1/21 of 360 days worth of shift would be 17 days.
It seems like a shift of the equinoxes by 13 days since the creation of the Tuzan Calendar would be easily noticable.
I am guessing the eclipses of Yael would be uneffected by this precession?
Or am I misunderstanding...
No, you aren't misunderstanding. MacDonald just made that number up, and it contradicts canon. The canon article mentions other sources of error in the calendar, but not precession. Accordingly, I left it out of my model. I did include a precession of the lunar orbit - that's a physical necessity (unless the laws of gravity and conservation of energy are repealed). But that doesn't effect the calendar.
Of course, the canon article has errors in it, too. There is another thread on the topic, but for example the statements about eclipses are just plain wrong.