I think that time is simply a convention for keeping track of events. There's nothing magical about it. We need to be able to order different events on a time line. Einstein has pointed out that this isn't as simple as it seems. Say we observe two supernovas explode, Supernova A at 6 PM PST and Supernova B at 6:01 PST. We can say that the event of supernova A preceded the event of supernova B or can we? If B is further away than A, and since light travels at a finite velocity, the event B may have preceded the event A. So in order to keep an accurate record we must take into account how far away an event that we observe is and how long it takes for light to travel to our observation point. Also we have to take into account things like gravitational fields which might bend the light as it travels to our observation point. This is what Einstein's theories of Special and General Relativity are all about. They make our record keeping completely accurate compared to the Newtonian system. However, it is still about record keeping.
Before the universe started, did time exist? That would depend on whether there were any events. If there were no events, then time would be useless. On the other hand, if there were events, then time would be useful for ordering them.
Space is similarly a system for ordering locations. Whatever geometry one wishes to use, we should be able to determine with complete accuracy the location of some object. Again Einstein's theory took more into account than Newton's so he was able to determine location with greater accuracy. Before the universe began, if there were no objects, then the concept of space as a way to measure location would have been useless and meaningless.
One can imagine time existing without space. In a non-physical realm, there is no location hence no space. However, there may be non-physical beings that do things from time to time, hence events which would need to be ordered for the historical record. Therefore, there would be a need for time.
Therefore, I believe that both time and space are merely conventions. At time = 0, we had the big bang, and at some later time there may be the big crunch. I think the universe will have a finite time span, and its whole history can be ordered with respect to time.
So what is eternity? Eternity is the timelessness that existed before the universe began plus the time of the universe's duration plus the timelessness that will exist after the universe ends. Since time is merely a convention, the universe really exists in or is embedded in a state of timelessness. Individuals are born and die. Stars and solar systems are born and die, and the whole universe was born and will probably die, but these are all events which can be ordered in time. Other than that, we exist in a state of eternity, but for a finite time span.
One might ask where in space did the universe begin? It began at a point, the point of the big bang, but where was that point. Is that point the center of the universe? Since space is only a meaningful concept in terms of measuring location in the physical universe, the point of the big bang is meaningless with respect to what preceded it, but not meaningless in terms of what came after it, namely the universe itself. I think it is a meaningful question to ask where in space is the point where the big bang started? It would be the origin of space and the origin of time.
What about the question is there anything beyond the universe? Again since space only has meaning in terms of the universe, it is a meaningless question. One could say there is nothing there, but whether or not there is nothing there is unknowable. What we do know is that the universe is expanding roughly at the speed of light so that new space is being created all the time and has been created ever since the Big Bang.
