Exodus 11:4

ויאמר משה כה אמר ה AND MOSES SAID, THUS SAITH THE LORD — Whilst he was still standing before Pharaoh this prophecy was spoken to him, because after he left him, he did not see his face again (Exodus Rabbah 18:1). כחצות הלילה means, WHEN THE NIGHT IS DIVIDED (i. e., when midnight arrives). כחצות is an infinitive with prefix, of similar form to, (1 Kings 18:36) בעלות, “when went up” and (Psalms 124:3) “when their wrath was kindled (בחרות)”. This is its literal meaning which fits in with the sense, for חצות is not a noun, being another form of חֵצִי (when the meaning would be about midnight I will go out etc.”, for this does not fit in with the sense, since God would certainly have stated exactly when He would do this). But our Rabbis took it in this sense and explained it as meaning “about the middle of the night”, and they asserted that Moses said כחצות “about midnight”, which implies near to it — either just before it or just after it, and he did not say בחצות “at midnight” (as God had bidden him say) because he feared lest Pharaoh’s astrologers might make a mistake as regards the time when the slaughter of the firstborn actually took place, thinking that it was a little earlier or later than midnight, and would say, in consequence of this error, “Moses is a liar!” (cf. Berakhot 4a).

וַיֹּ֣אמֶר מֹשֶׁ֔ה כֹּ֖ה אָמַ֣ר יְהֹוָ֑ה כַּחֲצֹ֣ת הַלַּ֔יְלָה אֲנִ֥י יוֹצֵ֖א בְּת֥וֹךְ מִצְרָֽיִם׃

And Moshe said, Thus says the Lord, About midnight will I go out into the midst of Miżrayim: