לרפואת אסתר רות בת נעמי שרה
There is a very interesting machlokes between Rav Zera and Rav Ashi as to whether Mohe said that he didn't know when midnight was so he said "about midnight". Rav Zera said that Moshe said that he didn't know so he said "about midnight". Rav Ashi said that he knew and said "כחצות" at midnight of the previous night and he said "Tomorrow [at the hour] like the midnight of tonight, I will go out into the midst of Egypt".
R. Zera says: Moshe certainly knew and Dovid, too, knew [the exact time of midnight]. Since Dovid knew, why did he need the harp? That he might wake from his sleep. Since Moshe knew, why did he say כחצות - 'about midnight'? — Moshe thought that the astrologers of Paroh might make a mistake, and then they would say that Moses was a liar. For so a Master said: Let your tongue acquire the habit of saying, 'I know not', lest you be led to falsehoods [lying]. R. Ashi says: It was at midnight of the night of the thirteenth passing into the fourteenth [of Nisan], and thus said Moshe to the Jews: The Holy One, blessed be He, said: Tomorrow [at the hour] like the midnight of tonight, I will go out into the midst of Egypt.
This gemara needs explanation.
1] How can Moshe have said "I don't know" when he did know???
2] What is the point of dispute between R' Zera and R' Ashi??
There are two types of knowledge: 1] Personal knowledge of people of a higher caliber. 2] Common knowledge that is accessible to larger numbers of people. It is correct to say "I don't know" when one's knowledge is only the personal knowledge of a great person. This is because "knowledge" is a term that applies to what people in general know. Thus, explains R' Zera, Moshe was telling the truth when he said "I don't know" about the מכה being at EXACTLY חצות. This very special point of time was not perceptible by the astrologers of Paroh and they would have accused Moshe of lying had he said a definitive "חצות".
R' Ashi understood that the astrologers were not going to start up with Moshe at this point, so he wasn't concerned with their reaction. Therefore, he could already get up at חצות of the previous night and tell the Jews that at this very special time tomorrow there was going to be a great מכה. It wasn't just the power of חצות לילה of the 15th of Nissan when the מכה actually occurred but was already present בכוח - in potential, at earlier times of חצות. This certitude about the miracle gave the Jews the faith to eat their Pesachim with the knowledge that the geulah is about to happen.
For more insight into this gemara see the sources quoted by the sefer עבודה ברורה on Brachos - מילואים סי' ד.
