Tempus sine aliqua mobili mutabilitate non est, in aeternitate autem nulla mutatio est
There is no time without change, but in eternity there is no change

Augustine, City of God, XI, vi

In aeternitate stabilitas est, in tempore autem varietas: in aeternitate omnia stant, in temporalibus accedunt, alii succedunt
In eternity there is permanence: in time variety.
In eternity all things stand still; among temporal things some come, others follow.

Augustine, Sermons, CXVII, 10(vii)

Aeternitas igitur est interminabilis vitae tota simul et perfecta possessio
Eternity, therefore, is the complete and perfect possession of unending life all at once

Boethius, Consolations of Philosophy, V,6,ll.9-11

Time is the passage from future potentiality through present actuality to past immutability.