Responsa על ראש השנה 40:25

I also asked [in my dream] about our uncertainty regarding the three Talmudic statements of Rabbi Zeira (Rosh Hashanah 20b:4) on the calculation of the calendar. First, that the night and the day need to belong to the new month. Second, that if the molad, the lunar conjunction, occurs before midday, the moon will certainly be visible close to sunset. Third, that the moon is invisible for twenty four hours [between the lunar cycles]. We have encountered the fundamental secret of the calendrical calculations, as elucidated by R. Zerachia¹see Hamaor Hakatan on Rif Rosh Hashanah 5a . It was on a Monday evening, the first day of the month of Tammuz, when I asked whether it is better to rely on the interpretations offered by Rashi²Rashi on Rosh Hashanah 20b, where he gives his own understanding of the calendrical calculations, and cites R. Saadia Gaon, or whether the better interpretation is that of the great Rabbi Zerachia.