How many years between Genesis chapter15 and Exodus chapter 12?

A.Using Bishop Ussher's dating, the confirmation of the Abrahamic Covenant in Genesis 15 vv.5 & 18 was in 1876BC, and the exodus of the enslaved Hebrews from Egypt in Exodus 15 was in 1446BC, thus making it a period of 430 years.

However, Genesis 15:13 also says Abraham's descendents would be enslaved for 400 years:-

Gen 15:12-13 (HCSB)

Gen 15 (v.12) As the sun was setting, a deep sleep fell on Abram, and suddenly great terror and darkness descended on him. Gen 15 (v.13) Then the LORD said to Abram [later renamed Abraham], "Know this for certain: Your offspring will be foreigners in a land that does not belong to them; they will be enslaved and oppressed 400 years.

This difference of 30 years between 400 and 430 years just means that they were not enslaved for the first 30 years.