First, I'd suggest this one for current calculations:
http://www.numachi.com/~ccount/hmepa/
For the date in 341 BCE, I'd be skeptical of any exact date. Trying to align the Olympiads and the lunar cycles of any Ancient calendar is fraught with complications. https://en.wikipedia.org/wiki/Attic_calendar?wprov= Gamelion, the month of his birth according to Laertius, falls in January to February:
"And he was born as we are told by Apollodorus, in his Chronicles, in the third year of the hundred and ninth Olympiad, in the archonship of Sosigenes [342/1 B.C.], on the seventh day of the month Gamelion [January/February], seven years after the death of Plato.