According to mainstream Christianity when did time begin?
According to mainstream Christians, time commenced at the moment of the creation of the physical universe which was created by God as explained in Genesis.
Time and God
Time could not exist without God, yet He does not need time. Humans require time to exist. My potential to be a different me becomes the actual me only through time. Over time, we change what we have, such as gaining knowledge or strength. And God can change what we are, like from a sinner to a saint.
These descriptions help address the question of how time began. “For by Him all things were created that are in heaven and that are on earth, visible and invisible.”5 Time may not be visible, but its effects are. Time is tightly tied to changing states of visible and invisible entities. Without time for the ant to decay, its essence would not change. Without time for a girl to learn the gospel, she would not have the opportunity to repent of her sin and trust Christ. Thus, time, space, and matter either all exist together or none exist.
When Time Began
Genesis 1:1 says, “In the beginning God created the heavens and the earth.” Clearly, no created entities existed before that moment. And without material or immaterial entities, how could anyone notice the passage of time? So, it looks like “the beginning” marked the first moment of time.
One could make a similar argument about space. Without space for material like clocks to inhabit, then no clocks could exist to mark the passage of time. And God did not create space (“the heavens”) until Genesis 1:1.
Another argument suggests the same conclusion. One of God’s first acts of creation was to invent a giant device to mark time—a spinning earth near a light source that delineated evening and morning. Hebrews 11:3 says, “The things which are seen were not made of things which are visible.” Scripture doesn’t record any things or events prior to the beginning, and time is defined by changes in the state of things. Apparently, prior to the creation week, there was only God, perfect in His timeless changelessness—just as He still is and will forever remain.6 Thus, the Bible suggests there was no time before the beginning of creation.
The concept of time has to take into consideration the the cosmology of the whole universe, yet not that our day revolves around a 24 hour day!
The problem of time is one of the most difficult and most keenly debated in the field of natural philosophy. To arrive at a satisfactory orientation in regard to this discussion, it is important to distinguish two questions:
What are the notes, or elements, contained in our subjective representation of time?
To what external reality does this representation correspond?
Time as a subjective concept
As to the first question, philosophers and scientists in general agree in this: that the notion, or concept, of time contains three distinct ideas fused into one indivisible whole.
First there is the idea of succession. Every mind distinguishes in time the past, the present, and the future, that is parts which essentially exclude simultaneity and can be realized only one after the other.
Again, time implies continuity. Speaking of events here below, in our own life, we cannot conceive the possibility of an interval of duration, however short, in which we should cease to grow older, or in which moment should cease to follow moment. The march of time knows neither pause nor interruption.
Lastly, a continuous succession cannot be a continuous succession of nothing. Therefore the concept of time represents to us a reality the parts of which succeed each other in a continuous manner. It matters little here whether this reality is purely ideal, or is realized outside of us, for we are dealing only with the concept of time.
Such are the three essential elements of the subjective representation. From these considerations it appears that the question of time belongs to the domain of cosmology. By reason of its character as continuous, successive, divisible, and measureable, time belongs to the category of quantity, which is a general attribute of bodies, and cosmology has for its object the essence and general attributes of matter.
Cosmology is a branch of astronomy involving the science of the universe’s origin.
Origin of cosmology
The word itself is of recent origin. It was first used by Wolff when, in 1730, he entitled one of his works "Cosmologia Generalis" (Frankfort and Leipzig). In this treatise the author studies especially the laws of motion, the relations that exist among things in nature, the contingency of the universe, the harmony of nature, the necessity of postulating a God to explain the origin of the cosmos and its manifestation of purpose. Because of the advance the natural sciences were then making, Wolff omitted from his philosophic study of nature the purely scientific portion which till then had been closely allied with it. The cosmology of the ancients and especially of Aristotle was simply a branch of physics. The "Physics" of Aristotle treats of corporeal beings in as far as they are subject to motion. The work is divided into two parts:
General physics, which embraces the general principles governing corporeal being. It treats of local motion and its various kinds; the origin of substantial compounds; changes in quality; changes in quantity by increase and decrease; and changes arising from motion in place, on which Aristotle hinges our notions of the infinite, of time, and of space.
Special physics which deals with the various classes of beings: terrestrial bodies, celestial bodies, and man.
It is the first part of this work that comes nearest to what we mean by cosmology. The Schoolmen of the Middle Ages, as a rule, follow the path marked out for them by Aristotle. Cosmological subjects, properly so called, have no reserved place in philosophical study, and are generally treated as a part of physics. In our own time, philosophers employ the words "cosmology" and "philosophy of nature" to designate the philosophic study of the corporeal world.
Aeviternity generally refers to time as experienced by the Angels, since they outside of time in correspondence to our physical universe. As to what is the difference between time, aeviternity and eternity, I will let St. Thomas Aquinas speak:
Aeviternity differs from time, and from eternity, as the mean between them both. This difference is explained by some to consist in the fact that eternity has neither beginning nor end, aeviternity, a beginning but no end, and time both beginning and end. This difference, however, is but an accidental one, as was shown above, in the preceding article; because even if aeviternal things had always been, and would always be, as some think, and even if they might sometimes fail to be, which is possible to God to allow; even granted this, aeviternity would still be distinguished from eternity, and from time. - Question 10. The eternity of God (Summa Theologiae)
Time as we understand it in Christian notion has a beginning and an end. Aeviternity of the Angels has a beginning, but no end. Eternity has neither a beginning or end!
Time to get my Tardis out! TARDIS is an acronym of "Time And Relative Dimension(s) in Space".