Time is so all-permeating in this universe that we take it for granted. The very laws of physics seem to have at least an element of time to it¸ going all the way back to the Big Bang. Everything is moving relative to some object in the universe, from the tiniest subatomic particles and photons to whole superclusters of galaxies. That movement requires time. After all, if no time existed, how can things move (look at how we measure movement in terms of speed: speed equals distance divided by time).
Does time have a beginning or end? Does it only exist as a product of the human thought process, as some physicists hold? Or does the “presence” of nothingness simply render time irrelevant but no less existent. The answer depends on what the definition of time is. The best definition of time I can come up with is as follows: A non-spatial phenomenon that allows for the potential for change, change in position or condition. Let's break this one down to make it more comprehensible.
A non-spatial (i.e. not bound to or measurable in terms of the three spatial dimensions: up-down, right-left, or forward-backward)
phenomenon: basically a phenomenon is a thing, place, event, or any other kind of occurance.
So another way of defining time is a "thing" that has no height, width, length (in the measuring stick sense) that allows something to change another thing's place or condition/nature. Debatable this definition may be, but that’s the definition I base this post on.
Change in position: You can move an ice cube across any of the three spatial dimensions: up-down, right-left, or forward-backward. However, what can not happen is the ice cube being in two different places at the same time a macroscopic object cannot occupy two different places at the same time. For it to occupy a different position, it needed something that enabled a change in that position. Although at the subatomic level, quantum physics does allow a subatomic particle two different spatial positions simutaneously, subatomic particles themselves do change positions, which still requires something to enable the change in the particle's position - which again requires the phenomenon called time.
Change in condition: For example, the ice cube can stay solid, turn to water, or turn to gas. Also, iron bars rust, living things die, and so forth. Before the iron bar and the living things existed, they were – ultimately – hydrogen nuclei in the cores of stars that eventually fused together. This was the first event in a long chain of events that lead to the creation of life and that iron bar. This is a change in the condition of the proton and neutron(s), if any, in that hydrogen nucleus – changing from “free floating” single nucleus to being part an iron atom’s nucleus; or in the case of a living thing, being part of the atom of a molecule of a living thing.
Another example of a change in condition is illuminating an object with a frequency of light it absorbs. If you use the “right” frequency of light, you can change the color of the object (in a manner of speaking). For if a green object happens to absorb all red light, then shining a red light onto that object makes that object appear black. In both cases, the atoms of those objects experienced changes in condition: the single hydrogen nuclei later incorporated into living matter and the iron bar, the green object reflecting green light, and later reflecting no visible light because the object absorbs red light. For the nuclei and that green object to change from red to green requires a change in the light spectrum hitting that object. That can happen only if time existed (i.e. enabled the change in condition/characteristic of the light reflected off the object).
I think something similar applies to the nature of time, particularly concerning the beginning of the universe. This directly concerns the question “Has time always existed or did it only begin with the birth of the universe). Some very respect-worthy physicists say that time itself did not exist before the Big Bang, but I disagree. After all, even if the pre-Big Bang singularity (more accurately, “the pre-Inflation” one) changed into the proverbial “hot primordial soup” of the universe, then that was a change in condition. Therefore, there had to be something to enable that change in condition, even if there happen to be some intermediary steps within the singularity itself that had to occur that we presently are ignorant of. The bottom line is that there was a singularity that went from having the original characteristic of the proto-universe (if it be called that) to the post-Bang universe with the “old” characteristics, and finally to the intermediate steps that lead to the universe as we know it today. No matter how or what caused the change in the singularity’s condition, it had to have some phenomenon that allowed for the potential for that change before the actual change in the singularity’s condition –into the universe as we know it. The changes in the position of all the matter and energy in the universe are self-evident, so I won’t go into it.
For these reasons, I think time is eternal, if defined as the nonspatial phenomenon that allows for changes in the position or condition of anything. You may disagree with this definition, but that’s your prerogative. I’m happy to hear a better (i.e., more complete, more comprehensive, and more coherent) definition of time, but for now this is the definition I am sticking with.