Machlokes Rebbi Yossi V'Rabbanan (Shabbos 87)
The Rabbanan say that Rosh Chodesh Sivan was on Monday, Rebbi Yossi says it was on Sunday. I was wondering what the molad was. The sefer Itim L'Binah (Yosef Ginzburg, Warsaw, 5649, reprinted in Israel) says that the molad Nisan for that year was 5-2-332 (5 days, 2 hrs, 332 chalakim), so then molad Iyar was 6-15-45 and molad Sivan was 1-3-838. He has a long discussion to explain why Moshe Rabbeinu made Rosh Chodesh Nisan on Thursday, however basically, Hashem showed him the new moon during the day, and told him to be Me'kadaish the Chodesh then. Anyway, according to this, Rosh Chodesh Sivan should be on Monday (the new moon was not yet visible on Sunday night). However, this molad is the molad benoni, the molad amiti is different. For Nisan of that year, the molad amiti was Wed. at noon, (earlier than the molad beboni) , however, the sefer does not discuss when the molad of Sivan might have been.