Musar su Genesi 12:4

Here is an instance that shows that the sequence in which the Torah reports events is not necessarily the chronological order in which things occurred. Remember that Abraham was seventy years old at the time of the covenant between the pieces in 15,18 (as we know on the authority of סדר עולם). Our sages also consider the count of the four hundred years that the Torah mentions in that covenant (15,13) as commencing with the birth of Isaac. When Exodus 12,40 mentions a period of 430 years that the Israelites "resided" in Egypt, our sages compute that period as having commenced at the ברית בין הבתרים "the covenant between the pieces," i.e. when G–d revealed Himself to Abraham in 15,1. [Rashi's commentary that this number includes the years spent elsewhere as strangers by Abraham and his descendants, may be based on the letter ו (and), before the word מושב in 12,40. Ed.]