That is VERY impressive, Eikadistes ! I certainly can't argue with any of your calculations, and, with that, I would concur that Jan. 20 seems to be a preferable *customary* date to celebrate Epicurus's birthday as set out in his Will.
From the articles below, it seems the Athenian calendar was much more fluid than we are accustomed to.
Some of that is laid out in this paper:
The Athenian Calendar
The Author provides a thorough introduction to the five (5) most commonly used calendars in Ancient Athens through the Roman Period. This paper presents an…
www.academia.edu
And this: