Is justice, then, variable and changeable? No, but the times over which she presides are not all alike because they are different times.
In Book III, Augustine works through a philosophy about history that allows for a law to be just in one time period and unjust in another. This idea accommodates the fact, for example, that righteous actions from the Old Testament might not be righteous later in history. For Augustine, justice has her temporal reasons, and the context of time plays a role in every situation.
Time never lapses, nor does it glide at leisure through our sense perceptions. It does strange things in the mind. See how time came and went from day to day, and by coming and going it brought to my mind other ideas and remembrances[.]
In Book IV, Augustine reflects on the memory of a friend who has passed away, and such thoughts trigger a long philosophy about time and perception. He explores the meaning of sorrow and the depth of loss. Before Augustine knew god, he had great difficulty making his peace with or even accepting death, and time played tricks on his mind. He acknowledges that time seems curious and later devotes entire books to exploring the concept.
Therefore, let the heart see that all time past is forced to move on by the incoming future; that all the future follows from the past; and that all, past and future, is created and issues out of that which is forever present.
In Book XI, Augustine dives deep into the question of time. From many aspects, he explores the intersection of humans experiencing time and wonders what time means as it relates to god. In general, he believes that god is the eternal present. Only humans think of experience in terms of past and future. He asks questions, forms theories, and continues to praise the lord as he writes about what he calls the temporal medium.
Your years are but a day, and your day is not recurrent, but always today. Your today yields not to tomorrow and does not follow yesterday. Your today is eternity.
Book XI focuses on the concept of time. Augustine tries to imagine what time means to god and here postulates that god exists in an eternal present moment that exists outside the human boundaries of time. He writes about the past and future, about speaking a line of verse slowly and quickly, and about how time might be defined by the movements of the sun, moon, and stars.