The present day calendar was directly adopted from the calendar of Meton in Athens. It is not even mentioned in the Gemara.
So the best idea is to go by the actual molad [conjunction].[As you can see brought in Tosphot in Sanhedrin page 10].
The Gemara does mention that "nowadays we know the time of the new moon" but does not say anything about a calendar. It could be referring to calculations already known in ancient Babylon about what time the "molad" [conjunction of the moon and sun occurs].(not to some hypothetical calendar that was used only in Athens)
Just to be clear --I am not arguing if the court [Sanhedrin] has the right to adjust the date of the new moon. (This can also be argued based on the Gemara in Sanhedrin page 10. But it is not what I am talking about here.) Here I simply mean that the Sanhedrin did not in fact adjust the calendar. If they would have you would except the Gemara to have mentioned it somewhere.
So the best idea is to go by the actual molad [conjunction].[As you can see brought in Tosphot in Sanhedrin page 10].
The Gemara does mention that "nowadays we know the time of the new moon" but does not say anything about a calendar. It could be referring to calculations already known in ancient Babylon about what time the "molad" [conjunction of the moon and sun occurs].(not to some hypothetical calendar that was used only in Athens)
Just to be clear --I am not arguing if the court [Sanhedrin] has the right to adjust the date of the new moon. (This can also be argued based on the Gemara in Sanhedrin page 10. But it is not what I am talking about here.) Here I simply mean that the Sanhedrin did not in fact adjust the calendar. If they would have you would except the Gemara to have mentioned it somewhere.
