To have an accurate timeline it must be understood that the events recorded in the Bible do not always happen in the order preserved by chapter number or book, they need to be understood where they actually occur by the historic events being described.
One of the chapters out of order is Exodus. 18. The first key to when this event occurred is found in verse 5 of this chapter. “And Jethro, Moses’ father-in-law, came with his sons and his wife to Moses in the wilderness, where he was camped at the mountain of God.” Moses reached the campsite at the mountain at the beginning of Chapter 19.