Months names are all one off?

[Koen G]

If it was quite a bit after ~1425, then we must conclude that the month names in the Zodiac pages were not written by the original author, but guessed by some later owner.  That was my whole point.

Isn't it generally assumed that the month names were written later? Both after the early 15th century and later than the German-is marginalia of You are not allowed to view links. Register or Login to view. etc.