Aeviternity differs from time, and from eternity, as the mean between them both, because time has "before" and "after"; aeviternity in itself has no "before" and "after" (which can, however, be annexed to it); while eternity has neither "before" nor "after," nor is it compatible with such at all.
Eternity is the measure of a permanent being, God. In so far as anything recedes from permanence of being, it recedes from eternity.
But the being that is measured by eternity (i.e., God) is not changeable, nor is it annexed to change.
Now some things (like us) recede from permanence of being, so that their being is subject to change, or consists in change; and these things are measured by time, as are all movements, and also the being of all things corruptible.
Angels recede less from permanence of being, inasmuch as their being neither consists in change, nor is the subject of change; nevertheless they have change annexed to them either actually or potentially.
That is, angels have an unchangeable being as regards their nature with changeableness as regards choice; moreover they have changeableness of intelligence, of affections and of places in their own degree.
Therefore angels are measured by aeviternity which is a mean between eternity and time.