When did Zipporah, Gershom and Eliezer leave?

I’ve been reading through Exodus again, and as always, new things stood out to me. One of them is in Exodus 4 and 18.

Jethro, Moses’ father-in-law is visiting the Israelites in the desert after they have left Egypt. It isn’t just him though, he has brought Moses’ wife and sons with him.

In Exodus 18:2-3 (NIV), it says:

After Moses had sent away his wife Zipporah, his father-in-law Jethro received her and her two sons.

Exodus 18:5-6 continues on:

Jethro, Moses’ father-in-law, together with Moses’ sons and wife, came to him in the wilderness, where he was camped near the mountain of God. Jethro had sent word to him, “I, your father-in-law Jethro, am coming to you with your wife and her two sons.”

So it explains that Moses did send them away and that Jethro was returning with them, but when were they sent away in the first place?

In Exodus 4:20, it says:

20 So Moses took his wife and sons, put them on a donkey and started back to Egypt.

Obviously Moses took them with him when he went back to Egypt. So it must have been at some point after he got to Egypt that he decided to send them away. It makes sense to do so in order to protect them when he realised how challenging Pharaoh was and the challenge they would face in leaving. There are a range of other possible times that it could have happened though, it could have even been as early as when he met Aaron on the way back to Egypt.