So the Wrath passed over on the 15th at midnight? How odd...that is not the 14th
I think this will be easier to follow if we leave the days of the month aside for a moment, as the natural tendency is to think in modern western terms.
Let's start on Tuesday, just to illustrate. We will speak in modern terms for now.
Tuesday evening, while it is still light out, say about 5 pm, the lamb is slaughtered, and the blood put on the doorposts. Tuesday after dark, say 9 or 10 pm, the lamb is roasted and eaten. It is finished by Wednesday morning.
In the meantime, during that same night the "wrath" goes through Egypt. Time-wise this would be the midnight between Tuesday and Wednesday.
The schedule is clear. The only question is how one identifies the days.If we call Tuesday until nightfall the 14th, and the 15th starts on Tuesday at nightfall, everything falls into place.
The instructions for the festival that appear a few verses later make this clear:
18 In the first month you are to eat bread made without yeast, from the evening of the fourteenth day until the evening of the twenty-first day.
Chair