I don't know if this helps, but in my catechumen class, my priest explained it this way:
Time (or Space-Time) has a definite beginning and a definite end. It is our realm, where the universe and all matter exists. Time allows movement, so things can change. Within time, we are trapped within the Present moment - the Future is unknown and the Past is fixed. God must be outside it because he created it, or rather, it exists within him. It can be diagrammed this way:
Then there is Eternity (which is a created realm, just like Time), where the angels and departed souls live. It has a beginning but no end. Since there is no time, there is no movement, and therefore no change (an angel's allegiance to God is fixed and cannot waver, for instance).
The third realm is Ever-Existing, which is the realm of God's essence. It has no beginning or end. It is uncreated, motionless, and changeless. (This is what we mean by "Ages of Ages", or in the Western Rite, "World Without End".)