Jewish scholars long ago realised that there is a problem with the number of years the Israelites spent in Egypt. For this reason, they decided that the 430 years was not just the period in slavery, as reported in Exodus, but that it started at the time Abraham received the promise, a revision that nicely fits in with 215 years. This new tradition was known to Paul, who records in in Galatians 3:16-17.
From a historical perspective, these contradictions and harmonisations are not really so important. There is no extra-biblical evidence that the Israelite people were ever in Egypt, and the respected Israeli archaeologist, Israel Finkelstein, says that over 90 per cent of scholars do not believe that the Exodus from Egypt ever happened, as described in the Bible. Lawrence E. Stager, author of Forging an Identity: The Emergence of Ancient Israel (The Oxford History of the Biblical World) says the evidence from language, costume, coiffure, and material remains suggest that the early Israelites were a rural subset of Canaanite culture and largely indistinguishable from Transjordanian rural cultures as well. They did not exodus from Egypt and there was no unified conquest of the Canaanites. Thus the Israelites, as a national group, never spent any time enslaved in Egypt, although small numbers of individuals may well have done so.