Is God timeless, or everlasting? Or, is God to be seen as someone who stands outside of time, or someone who interacts in time while not bound by its limitations?
Traditionally, God is seen as timeless, that is, God stands outside of time. In the sense that there cannot be any change in God, i.e. God is immutable, God in His essence must be timeless. But if we just say that God is timeless, and continue no further than that, such would pose the problem of how God can interact with people. Something that is outside of time, if it does not come into time, cannot interact with things in time. After all, interaction in time requires time to be shared between the subject and the object of an interaction, and a timeless being is unable to "share time."
It should come as no surprise when certain Baptist theologians in England in the 18th century decided to attempt consistency between their doctrine of God and their understanding of God's decrees. We call these theologians Hyper-Calvinists. Although it can be debated whether John Gill was a Hyper-Calvinist, what is common between any high Calvinist and the hyper-Calvinists of that era is their immenantization of God's decree. For since God is simple, God's will is one, and God is timeless, therefore God's decree must be ever present and ever true at all times. God decrees to justify the elect, therefore justification of the elect must be eternal as God's decree is eternal. One can call that kind of reasoning "Rationalism" if we so desire, but that tells us nothing about the path of reasoning the hyper-Calvinists take to reach the doctrine of eternal justification.
The decision to immenantize God's decree comes from viewing God as solely timeless, and then trying to resolve the tension between God's timelessness and God's decree which seems to be progressively executed, with the tension solved by denying the progressive execution of God's decree. Of course, one could say that their problem is trying to resolve a tension that should not be resolved, but my point in giving this example is to show that seeing God as merely timeless is not without its potential problems.
A greater problem for the issue of timelessness comes, ironically, in practice. Specifically, it concerns the doctrine of prayer. What exactly happens when we believers pray? If we say God hears our prayers and answers then, that implies that God interacts with us and works in time. But a timeless being cannot do that. A timeless being can institute "timed acts" to happen at best, so we can say that God through foreknowledge knows what and when I will pray and thus He instituted an act X to occur time Y after my prayer to give the illusion of an interaction. But this is not genuine personal interaction, but rather more akin to an automated scripted response, where a computer is told what to do and what responses to give when a certain command line or request is relayed to the system. Does the one true God who is portrayed in Scripture as being involved with His people and delighting in them sound like an automated response script to you? I hope not! But this is probably the best idea of God's "interactions" if God is indeed timeless in every respect.
If God in all three persons interact in time with us His creatures, then all three persons of the Trinity must interact in time, although they are certainly not bound to the limitations of time. It is only when we see God as interacting in time that the idea of prayer as being that of a creaturely petition to God in a genuine covenantal interaction makes any sense. But if God interacts in time, then we must say that God in His acts is not timeless. Rather, "eternity" in this case means "everlasting." God's works, which is to say God ad extra, is not timeless but rather everlasting.
Since God in His work is everlasting, that means that we can speak of "eternity past" and "eternity future." We can speak of the persons of the Trinity in a dynamic fashion, and thus the eternal Covenant of Redemption is constituted not in a timeless "now," but in the "eternal time" before time actually begun. While we must always maintain that God is outside of time in His essence, we must maintain that God in His works is situated in "eternal time," which is to say "time" as a series of progressions rather than created time t, the 4th dimension in space-time physics.
We must remember that the overwhelming imperative for this argument is to preserve the reality of prayer and real actual communion with God. These things are to be non-negotiable realities of the Christian faith, and any theology that cannot account for them is to be rejected immediately. If one were to preserve timelessness as being not only pertaining to God in His essence but to the persons of the Godhead, then the onus is on that party to tell us how prayer can be real if the persons of the Trinity are all timeless persons. In my opinion, it can never work, but I would certainly be interested in anyone who wants to attempt such a case!