Jamieson, Fausset and Brown on Exodus 12:40

40. the sojourning of the children of Israel ... four hundred and thirty years--The Septuagint renders it thus: "The sojourning of the children and of their fathers, which they sojourned in the land of Canaan and in the land of Egypt." These additions are important, for the period of sojourn in Egypt did not exceed two hundred fifteen years; but if we reckon from the time that Abraham entered Canaan and the promise was made in which the sojourn of his posterity in Egypt was announced, this makes up the time to four hundred thirty years.