then Rosh Chodesh of Tishrei would arrive 3 days before Rosh HaShana. A lunar month is 29½ days, and therefore a 12-month period should have 6 full months of 30 days and 6 deficient months of 29 days. If there were 9 full months, this would cause subsequent months to start 3 days after Rosh Chodesh. This in turn would lead people to deride the rabbinical court and accuse it of declaring the new month whenever it wanted, without regard for the astronomical facts. The Talmud explains its question: Now, too, when there are 8 full months in one year, Rosh Chodesh of Tishrei will arrive two days before Rosh HaShana, which will also cause people to disparage the court.
כדאמר רב משרשיא, כגון שהיתה שנה מעוברת, הכא נמי כגון שהיתה שנה שלפניה מעוברת, ועיבור שנה חודש. דל ירחא מליא בהדי ירחא חסירא, ואכתי חד יומא! לא אדעתייהו דאינשי.
The Talmud answers: It is as Rav Mesharshiyya says, with regard to a different question: This is referring to a case where the year before was a leap year. Here, too, one can answer that it is a case where the year before it was a leap year, and the extension of the year was accomplished with an added month of 29 days. Accordingly, remove a full, 30-day month from the calculation of the current year corresponding to the extra deficient, 29-day month of the previous year, and the result is that only 37-day months have an impact on the appearance of Rosh Chodesh relative to the following Rosh HaShana. The Talmud asks: But even in a year with only 37-day months Rosh Chodesh would still precede Rosh HaShana by one day. The Talmud answers: People do not pay attention to a difference of one day, and therefore they will not deride the court.