Phizzy:
So even when a seemingly specific period like "430 years" or whatever is spoken of, this is often for literary reasons, and rarely accurate.
Such details are often initially allegorical or have some other esoteric meaning, but nutters later take them literally. Because details in Exodus 6 and 12 contradict other details in Genesis 12 about the duration the Israelites would supposedly be in Egypt (not a historical event), the alternative tradition developed that ‘Israelite slavery in Egypt’ ‘really’ started when Isaac was briefly mistreated by Ishmael (though in the story there weren’t any Israelites before Jacob). Of course, neither version is actually true though, and the Israelites were actually originally a Canaanite tribe.