When was the Passover Lamb selected?

Question

Exodus 12:3-6 says:

3 Tell the whole community of Israel that on the tenth day of this month each man is to take a lamb[a] for his family, one for each household. 4 If any household is too small for a whole lamb, they must share one with their nearest neighbor, having taken into account the number of people there are. You are to determine the amount of lamb needed in accordance with what each person will eat. 5 The animals you choose must be year-old males without defect, and you may take them from the sheep or the goats. 6 Take care of them until the fourteenth day of the month, when all the members of the community of Israel must slaughter them at twilight.

These verses seem to conflict with how Exodus 12:21 says that the Passover Lamb was chosen and slaughtered on the same day (which was the night of the death of all firstborn Egyptians).

21 Then Moses summoned all the elders of Israel and said to them, “Go at once and select the animals for your families and slaughter the Passover lamb.

Verse 21 seems to say that the lambs are being chosen on the same day they are killed, while verses 3-6 say that the lamb is selected four days prior to their slaughter.

Is there any way to reconcile these differences?

Answer 1

Rav Hirsch says that this means, carry out the commands that had been given on the first of the month (from the beginning of perek 12) at the times specified in those commands. This implies that this command (12:21) was said on the tenth.

הפסח evidently presupposes that this idea was already known to the people, so they must have already been informed of the whole laws of Pesach and Matzos. It is possible and probably that these words were said to the people on 10th Nisan, the day on which they were to carry out the command given to them on the 1st

Rashi implies this when he says that those who have sheep are to take them from their own flocks and those who do not are to buy them (now).