If one was alive in 399BC how would they know what year it is? Nobody had even heard of Christ-he wasn’t born.
Maybe I missed it, but I don’t see that referred to in what he wrote. Of course, they weren’t using the Gregorian calendar back then, but they did have types of calendars.
The Gregorian calendar was instituted in October of 1582 by the Catholic church under Pope Gregory XIII.
Britain and her colonies did not adopt it until September of 1752. Russia, under the new Soviet regime, didn’t start using it until February of 1918.
Before then the Julian calendar had been in use since 45BC by edict of Julius Caesar.
Greece didn’t officially become a Christian nation until Constantine by the Edict of Milan in 313AD. At some point afterward they adopted the Julian calendar. Greece did not switch to the Gregorian calendar until 1923. Before the Julian calendar was adopted the Greeks had many calendars.
https://en.wikipedia.org/wiki/Ancient_Greek_calendars