What is the Bible's oldest Messianic prophecy?

Some Messianic prophecies occur near the beginning of the Bible, such as Deuteronomy 18:15 (or even Genesis 3:15). What I want to understand is which Messianic prophecy do scholars believe was actually *recorded* first? Does, for example, the Immanuel prophecy of Isaiah 8:23-9:1 predate Deuteronomy 18? If that question is too difficult, due to questions of authorship and redaction, can we ask **What is the earliest date for which historians are confident Messianic prophecy existed in Israel?**