Question: Does the prolonging of life in Daniel 7:12 mean that unbelievers are in the millennium?
Daniel 7:12 is referring to God “preserving” and “melding” the previous 3 empires (first described in Daniel 2) into the final world ruling empire – Rome. The purpose and fate of these empires is unfolded in chapter 7, verse 12.
So we can see that this is not talking about “unbelievers”in the millennium, but rather it demonstrates that the 3 previous world empires will be granted an “extension of life” (Dan 7:12) and become part of the final fourth world dominating empire – the revived Roman empire. This is the final empire in place when the Lord Jesus returns! (Dan 2:31-45; Rev 13:2)
The ultimate fate of this final empire is presented in Daniel 2:35! It is utterly destroyed…”so that not a trace of it was found” (v.35).
Your question about “unbelievers in the millennium” is addressed, though, in Matthew by the Lord Jesus Himself (Matt 24-25, the Olivet Discourse) and particularly in Matt 25:31-46 — the passage known as the “Sheep and Goat’s Judgment” or the “Judgment of the Nations”. It is a very sobering read!