Craig and Moreland both argue against a strong doctrine of divine immutability. They use the following example:
1. God is timeless only if He is immutable. (Definition)
2. God is immutable only if He does not know what time it is now. (Premise)
3. If God is omniscient, then He knows what time it is now. (Premise)
4. God is omniscient. (Premise)
5. Therefore, God is not timeless. (From 1 - 4)
(5) entails (6): God is not immutable.
I list (2) and (3) as premises, as opposed to definitions, because what they are assuming (and they are quite upfront about this) is an A-theory of time. If time is dynamic and the present is constantly changing, then in order for God to be omniscient, He must know what the present is. Given that the present changes, there is a change in God's knowledge, implying that God is not immutable.
I have to wonder whether this turns out to be an argument not against immutability, but rather against an A-theory of time. Consider this:
1*. God is immutable and omniscient. (Premise)
2*. God is only immutable and omniscient if He knows all times simultaneously. (Definition)
3*. If all times are simultaneous to God, then time is static. (Definition)
4*. Therefore, time is static. (From 1 - 3)
(4*), of course, is a description of a B-theory of time.
It's not that I'm trying to take a position on whether I prefer an A-theory or a B-theory. Rather, I point out that one's presuppositions with respect to the nature of God will ultimately prove determinative for one's view of the relationship between God and time. If an A-theory is incompatible with divine immutability (and it's arguably not), then a good argument for divine immutability will simply lead one to accept a B-theory.