In Shemot 11:4, Moshe tells Pharaoh that the plague of the 1st born will begin at approximately midnight.
Rashi explains that the reason Moshe said "approximately" is that if Moshe said exactly at midnight, and the astrologers calculate it differently, they would think that Moshe was a false prophet.
I read Sifsei Chachamim's explanation as to why Moshe would care what others thought. I still don't get it, though.
G-d is trustworthy, and He knows exactly when He will do things. I also assume that Moshe had full faith in G-d. So, with all these factors, why doesn't Moshe just state outright that the plague will happen exactly at midnight knowing full well that if the astronomers erred, they would just be mistaken?