[ADMIN EDIT: I will update this first post in this thread for what we conclude. I see Don calculates Jan 24, but last year Takis calculated January 20th, so it's not clear to me yet as I type this what the correct date should be.}
I went back and looked at a previous thread on Epicurus's birthday calculations and found this was the best source for Ancient > Modern calendar calculations:
So, Epicurus's birthday (7 Gamelion) "falls on" Thurs., Jan. 21 this year.
I realize trying to line up Ancient calendars with modern ones is a tricky business and involves some guesswork, but this one seems to use the best current scholarship (even though it's a neo pagan organization). I also found this particular calendar interesting in light of my recent delving into On Piety with its commentary on Epicurus's participation in all the festivals... which are noted on this calendar.
Even if only used for entertainment, enjoy!
*So, Takis and this calendar coincide:
7 Gamelion is "afternoon of 20 January until next afternoon 21 January."