From Vincent Cheung’s commentary on Malachi:
God first reminds the hearers of his immutability, saying, “I am the Lord, I change not” (v. 6). God’s attributes remains the same, and they will never change. He is not subject to any external influence, and he is eternal so that there is no before or after in his being, so that he does not change. His omniscience implies that he has no succession of thoughts, and therefore he does not change his mind. His knowledge and decisions eternally exist in his mind, and are not subject to alteration. Since he knows all, he does not gain knowledge, and nothing surprises him. Since he is eternally immutable and comprehensively perfect, he never becomes better or worse.
Cheung contradicts himself in the first sentence. God is immutable, meaning God has no “before’s and after’s” yet he is “reminding” “hearers”. Those sound like actions, in time, with “before’s” and “after’s”. Cheung seems not to be self-aware as to how the context of Malachi contradicts his claims about the meaning of the text.
“… so that there is no before or after in his being …” sounds like Platonism to me.