According to mainstream Christianity when did time begin?

Note: After further study I see that I mischaracterized Aquinas' position below. Thanks to those who have corrected me on this. I will leave my answer up. My personal opinion has not changed but I was mistaken to say this is normative for mainstream Christianity.

The normative answer was given by Thomas Aquinas in the Summa Theologica:

God sees all things in His eternity, which, being simple, is present to all time, and embraces all time.

See the above link for a more thorough discussion of Aquinas' view. There are certainly other viewpoints held by mainstream Christians, but Aquinas is the most authoritative. The idea is particularly important as a refutation of the Arian "heresy," in which God created Jesus at a particular moment -- which might be seen as the beginning of time -- rather than co-existing together with the Son and the Spirit from eternity.

For mainstream Christianity (generally speaking), time did not "begin." God exists in the eternal "now." Both God and time are eternal.

Addendum:

For scriptural evidence of time existing before the creation see Ephesians 1:4 -- "he chose us in Christ before the foundation of the world to be holy and blameless before him in love." To say that time did not exist when God 'chose us' make Him oblivious as to timing.

However, after further study, I must admit that Aquinas does not take this position.

Rather, appealing to both reason and to St. Basil, St. Thomas argues that both material things and time itself were formed when God created the universe, a process which St. Thomas argues was instantaneous. He says, “And so it is that holy Scripture proclaims the creation of things to have been effected in an indivisible instant; for it is written: ‘In the beginning God created heaven and earth’ (Gen. 1:1). And Basil explains that this beginning is ‘the beginning of time’.”source

According to mainstream Christianity when did time begin?

This is a more speculative question than maybe the OP realizes.

The premise of your question is that Christianity teaches that time had a beginning. I believe Christianity does not in fact teach this.

For me, to prove that Christianity teaches time had a beginning it would be necessary to show the scriptural evidence. I'm willing to stand corrected but I don't believe there is any scriptural evidence.

If Christians believe that time had a beginning it is probably the case they are absorbing the idea from modern scientific thought.

In some hymns there is the thought that God existed before time was. This is possibly a poetic way of describing the eternity before the first day of creation. But the hymn writer is not necessarily meaning that before the creation there was literally no time. When they write it they are referring to that which was before the first moment of creation on the first day of creation.

There may have been time literally backward for an eternity.

But what is certain is that God is not limited or bound by time or space. He sees all of time and all of space simultaneously. At all times and in all places he is fully present, not just in part, but in the fullness of his being. He is just as fully in the room where you are reading this as he is fully seated on the throne of heaven. He chooses to reveal himself in heaven and to hide himself in your room.

(For what its worth I think Christianity does not teach the angels existed before the first day of creation. To me, the whole emphasis of Genesis 1 is that all things were created in the first week, including the angels. Either way, I think it true that "Christianity" more likely teaches the angels were created the first week than they existed before the first week: there really isn't much Biblical evidence for that, though there might be some in poetic portions of scripture, such as Job or the Psalms. On the other hand, early Genesis is not poetry and should not be treated as poetry.)

Before God created the heavens and the earth the Father, the Son and the Holy Spirit loved each other with an infinite love. Those who believe there was literally no time before the creation, what are they saying? That the Three Persons loved each other but there was literally no time in which to express it? The idea there was literally no time produces such nonsenses so I'm inclined to think the idea (of no time) is itself a nonsense, and persuaded to think Dan Fefferman's statement is better:

"For mainstream Christianity (generally speaking), time did not "begin." God exists in the eternal "now." Both God and time are eternal."

Some scientists or philosophers who believe in God might teach that when God created the heavens and the earth then he also created time and space. But they get that from their own thinking, not so much from the scriptures.

Every measurement of time involves measuring a change in some physical quantity. It may be a number on a digital clock, a heartbeat, or the position of the Sun in the sky. In physics, the definition of time is simple—time is change, or the interval over which change occurs. It is impossible to know that time has passed unless something changes. - LibreTexts Physics - Chapter 2.3: Time, Velocity, and Speed

This has two major implications relating to Genesis 1:1 and OP:

Firstly, without matter and space there can be no measurements of initial position, velocity, mass, distance, etc. (because there would be nothing to measure) and therefore no measurement of change is possible, rendering the notion of time in such a system nonsensical. In order for time as we understand it to be conceived as having always existed, matter and space (physicality) would have to always exist as well or we would have to imagine how physical change could occur in the absence of physicality.

Secondly, God has declared that He does not change and this absence of change places Him in the category of timelessness which, according to our current understanding of time, also places Him outside of physicality.

If, as some suppose (follow the links here to Mi Yodeya), there existed some uncreated matter from which God formed heaven and earth in the beginning then there would have been uncreated space existing also in order to contain such matter and, therefore, there would have been uncreated time. In this paradigm, God's existence and creative act would have occurred "in time" and He therefore, existing and acting in time, would be subject to change. It is the responsibility of those holding this position to explain how God would remain unchanging while existing and acting in the midst of uncreated matter and space.

But God, in the act of creating space and matter, brought time into existence and, as creator, remains essentially outside of every aspect of physicality and is, therefore, changeless. In the beginning (time) God (force) created (action) the heavens (space) and the earth (matter). Prior to this there was just God.

As for the the angels who shouted for joy as God laid the foundations of the earth ... we know that at least some of the angels changed, they fell. This places them within the bailiwick of time and sets their creation at some point after Genesis 1:1. Genesis 1:2 begins God's 'setting' of the earth's foundations and so angels seem to have been created very, very early "in time", between Genesis 1 verses 1 and 2.

It also places the Christ "without whom nothing was created that has been created" squarely outside of time until His entrance into time in the womb of Mary.

• An assumption underlying this answer is that angelic beings are in some way constrained or effected by physicality or the byproducts of physicality (time) though they themselves are not wholly physical. I offer Daniel chapter 10, where one spirit being delays another spirit being for 21 days, as an evidence of this assumption.

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!

Time

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".