God is outside time. There still exists sequences of events in Heaven, so it does have some form of time, though it might not be connected to time in the material universe.
If you just consider the fact that there was a state where there did not exist a Kingdom of Heaven, and a state where the Kingdom of Heaven does exist, both juxtaposed by the Ascension, then you can see there exists some form of sequence at least in parts of Heaven.
Angels were not allowed into the Beatific Vision until they passed their trials. So there was a point at which your guardian angel did not participate in the Beatific Vision and a point at which he does.
We know purgatory has to have some form of time involved, since it is dogma that we must spend some period of time there unless we are saints or are condemned to Hell. Personally, I suspect Heaven and the material universe are at least logically connected (they share in logic, mathematics, problem spaces, complexity, etc.,). So it’s not like there is this clean break between the two realms. We can experience at least some of Heaven in these more abstract facets of our universe.
If I have to guess (and I do), I would argue that everything in the material universe points towards Heaven. Nothing God does is meaningless. For example, the material universe exhibits relativistic effects. It’s not like there exists one drumbeat of time that pushes the entire universe from one tick to the next. The passage of time depends upon acceleration and gravity. As you accelerate towards the speed of light, time diminishes towards eternity, though you can never reach it, since you are not a photon. I suspect Heaven is similar in that “lower” portions are more temporal and more in common with the material universe’s passage of time. Eternity is God, and only God and saints exist in that. So, in that sense, admission to the Beatific Vision is the spiritual equivalent of a physical object (not a photon) somehow achieving the speed of light, which should be impossible (though with grace, the spiritual equivalent is possible).
So at the very top you have eternity. That is the Holy Trinity and the place that only saints will ever go to be with God. Then Heaven expands out from the Holy Trinity. The further away from seat of God, the more temporal Heaven becomes, in a similar fashion as the material universe works with respect to light. God is a spiritual light, after all.
When we go to mass, the things we don’t see is our entering into eternity in some mystical way. I think when we live in grace, part of us already has a connection to that eternity with God, though it’s hidden from us.
It is true that we cannot change after death, but I am not sure that is necessarily because of eternity, but because we lack a physical body that would allow us to change. When we are resurrected, and God willing we are not thrown into Hell, then I think we will live in both places. Part of us will always be in eternity with God. But our physical bodies will still be able to exist in the physical universe that exists in the Eternal Order. So even there the situation with our living in grace here, and our participation in the mass, point to something in our future, where we will be completely in unity with the eternal, even though we will exist in time as well. But it will no longer be hidden from us either. We will have our spiritual faces towards the Beatific Vision forever, and our physical faces directed to whatever it is we feel like doing in the eternal universe to come, similar to how our guardian angels both constantly watch over us and also have their faces turned to God.
These are just speculations. I might be wrong about them, so take them with a grain of salt. Some of the issues about time in Heaven are explored by St. Thomas as well.
