Shabbat 88A

לרבנן שמונה חסרים עבוד.

and according to the Rabbis, they established 8 months that were lacking.

תא שמע, דתניא ב״סדר עולם״: ניסן שבו יצאו ישראל ממצרים בארבעה עשר שחטו פסחיהן, בחמשה עשר יצאו, ואותו היום ערב שבת היה. ומדריש ירחא דניסן ערב שבת ⁠— ריש ירחא דאייר חד בשבא, וסיון בתרי בשבא, קשיא לרבי יוסי! אמר לך רבי יוסי: הא מני ⁠— רבנן היא.

The Talmud cites another objection. Come and hear that which was taught in a baraita in the anthology called Seder Olam: In the month of Nisan during which the Jewish people left Egypt, on the 14th they slaughtered their Paschal lambs, on the 15th they left Egypt, and that day was Friday. From the fact that Rosh Chodesh of Nisan was on Friday, we can infer that Rosh Chodesh of Iyyar was on the 1st day of the week, and Rosh Chodesh of Sivan was on the 2nd day of the week. This is difficult according to the opinion of R' Yosei, who holds that Rosh Chodesh of Sivan was on Sunday. The Talmud answers that R' Yosei could have said to you: Whose is the opinion in this baraita? It is the opinion of the Rabbis. Therefore, this baraita poses no difficulty to the opinion of the R' Yosei.

תא שמע, רבי יוסי אומר: בשני עלה משה וירד, בשלישי עלה וירד, ברביעי ירד ושוב לא עלה. ומאחר שלא עלה, מהיכן ירד? אלא: ברביעי עלה וירד, בחמישי בנה מזבח והקריב עליו קרבן, בששי לא היה לו פנאי. מאי לאו, משום תורה?

The Talmud cites another objection: Come and hear from that which was taught, that R' Yosei says: On the 2nd day of Sivan, Moses ascended Mount Sinai and descended. On the 3rd day, he ascended and descended. On the 4th day, he descended and did not ascend Mount Sinai again until he was commanded along with all of the Jewish people. And the Talmud asks: How is it possible that he descended on the 4th day? Since he did not ascend, from where did he descend? Rather, this must be emended: On the 4th day, he ascended and descended. On the 5th day, he built an altar and sacrificed an offering. On the 6th day, he had no time. The Talmud asks: Is that not because he received the Torah on the 6th day of the month? Apparently, this baraita supports the opinion of the Rabbis.