(Hirsch; cf. Mekhilta; Megilla 9a). Since the 400 years (Genesis 15:13) were counted from Isaac's birth, and Isaac was born when Abraham was 100 years old (Genesis 21:5), the 430 years had to have begun when Abraham was 70 years old. Abraham was 75 years old when he left Charan (Genesis 12:4), so this was five years earlier. Some say that this is counted from the time of the Covenant Between Halves (Genesis 15:13), which occurred before Abraham left Charan (see Mekhilta; Seder Olam; Gra ad loc.; Rashi). Others say that Abraham began his life of wandering as a foreigner when he was 70 years old, and his lifestyle endured for 430 years (cf. Gra loc. cit.; Rambam). Others state that Canaan was under Egyptian domination, and therefore the Israelites and their ancestors had been under Egyptian domination for 430 years (cf. Torah Sh'lemah 421).
If we assume that 400 years elapsed between Isaac's birth and the Exodus, then, since Isaac was 60 when Jacob was born (Genesis 25:26), and Jacob was 130 when he came to Egypt (Genesis 47:9), the total number of years the Israelites were in Egypt was actually 210 (Seder Olam; Pirkey Rabbi Eliezer 48).
Josephus, however, states that the Israelites were in Egypt for a total of 215 years (Antiquities 2:15:2; cf. Septuagint; Pirkey Rabbi Eliezer 48). According to this, the 430 years were counted from the time that Abraham was 75 years old, when he left Charan. The 400 years would then have begun when Isaac was five years old.