From various sources we have a few sets of critical dates prior to Skynet turn-on and subsequent obliteration of most of mankind. The key ones are centered on Sarah Connor, her birthday, and age at T1.
From T1 we get an age for her of 19, and the dates of the world of the movie as 12-14 MAY 1984. This puts her birthday region between 15 MAY 1964 to 11 MAY 1965.
From T2 we get a range of ages for Sarah Connor in T1 with her age in T2 as 29: 17, 18, 19.
From T3 we get her age in T1 due to her headstone giving a birth year of 1959: 24 or 25.
Something is changing as times change! Now the Sarah Connor Chronicles gets a bit too much into the 'time travel' for fun and games area, and things really start to get messed up with time traveling Terminators and human agents mucking about with everything. What happens when timelines get all messed up?
The time they start to change originates with 2029 and the time travel use by Skynet and the human resistance. This then brings into play the major question of 'timelines': the course of events that happen after traveling back in time. The concept of a 'timeline' is a historical course of events that have a cause and effect relationship, in which events happen in a sequential order and follow on events are based upon the outcomes of the prior events. Thus, when you travel back into time, you change the ordering of events. At the point you (or any object) moves back into time, the future is no longer set in the timeline you are in. Here there are various paradoxes that arise, like killing your own grandfather and such, to the point where viewing time as a single flowing stream falls apart. That brings into question the possibility of time travel and what it means to 'time travel'.
Currently there are some theories that would indicate a unidirectional flow of time due to our current understanding of the basic forces at work in the universe. Part of magnetic and quantum theory, along with particle theory, posits that there are some forces that do *not* reverse direction with time. The primary one is undiscovered, so time travel stories are still free to play with the 'flowing stream' concept, even with all of its problems, but it does seem to have a mass, charge and otherwise possible 'fit' with all known theories and may even be mandated by some of them: the magnetic monopole.
A magnetic monopole is a single pole of a magnet. All known magnets have two poles with a flow of magnetic force between them, thus allowing time to reverse a direction of flow. Magnetic monopoles, however, have only one single direction (North or South) and reversing times flow does *not* reverse polarity for these objects. North remains North and South remains South. From that any particle that is deflected coming into a monopole field (say to the right of its direction of travel) will still deflect to the right if it is going backwards in time and not to the left. A discovery of magnetic monopoles would be a great boon to mankind for eliminating some problems of magnetic fields, but would destroy the 'flowing time' concept of time travel fiction. Other things like charged black holes also have a similar effect, so when going back into time a 'reversed' flow concept must be kicked out of the realm of what is possible as there are too many pointers against it.
That now leaves us with a form of quantum dislocation and changed timelines, or already existing timelines (of which there are many theories).
In a quantum dislocation in a single time line, there is a mass exchange (uptime to down) along with the energy to perform that exchange. Because this is a quantum shift, causality becomes a moot point as you are exchanging one possible configuration of a given set space for another: that is always possible but almost impossible to have happen. Once you do that, the space you *left* ceases to exist for all intents and purposes: it has annihilated itself to reconfigure a past time with a part of that possible future in it. By putting that future observer into the past, the one thing you can and must say is that the configuration that allowed that to happen is no longer viable: by having an observer with 'future' knowledge, the quantum effects spread out on a particle to particle interaction basis and the wave front of the new interaction is going at the speed of light. Once we see the Terminator appear, we know that the timeline it is in can no longer yield that future that sent it at the time, place, and position it was in when that happened. In theory Kyle Reese should have had ZERO probability of being able to be sent from THAT universe. In effect the moment of sending is a 'roll of the dice' gamble that you will get a future more amenable to your wishes, but it would always mean that the universe that sent it has now wiped itself out due to the observer effect.
That said, and without knowing the equipment used to do this, it might be possible that there needs to be a charge build-up to perform the transfer and that this was still going on after the rebels captured the base and sent Kyle on a hasty mission to stop the Terminator that was in the process of being sent. Trying to stop it might have wiped out the force or had other consequences, so while the machinery was still in the process of sending, it was put into a mode to take a second individual at a slightly different set of coordinates but at the same time period. From that you can get two individuals going back into time from one place, but that is the mechanical problem of time travel, and once the sending is *done* that reality ceases to be.
That is the direct quantum view without invoking alternate histories: a new history gets created with each transport back in time. You kill your own grandfather? So what? That universe that existed as a potential isn't there *any more* and it served its purpose to get you where you are. Want to get back to it? Sorry, its gone. It was possible to get to that universe before it sent you back, but once it did you can only get to a close approximation of it at best.
A second view is that of pre-existing alternate timelines that are very, very, very similar in configuration, but unconnected in some larger set of continua. Here shifting is not within one time line but from one to another, with an equal transfer of mass and energy each way. This has the bonus of having unlimited sending capacity... but doesn't change the circumstances of your time line one bit: you are mucking about in someone else's history.
Thus T1 would be set in a parallel time line, and Skynet (for its own reasons) is trying to either set up a bastion there or make it easier for itself in that time line to arise. Here the fun is that if you are sending simple things out to test the machine (say ball bearings) and there are multiple time lines discovering this and trying it out, they could be doing this to each other and for all the outside information they would think they are doing this in their own timeline! That slowly devolves to the previous case, however. So you like to stick with one time line as being the only originator for this stuff. Still, if you like messy work, then having five or six or ten or a hundred different time lines all sending things back into each other's past, you will also start to see a number of them disappear as that happens. That is a pain in the ass.
A third view is on the quantum travel is that at the point of introduction of the change, there is a bifurcation of the timelines (or actually a spreading spectrum of changes based on multiple different possible outcomes but for simplicity keep it at two) which is separate from the original one and will have no impact on it. This actually has a bonus in that if you test for it in the very short term (say 10 minutes) it will appear not to work in the originating universe but appear *to* work in one of the others. The time machine worked, hurrah! Lets send someone really far back in time... say, why didn't that work? Here the short term work is all done within the actual creation of the device, so it has all the appearance of working to the universe it came out in, while not working in the one that sent it. If you posit only *one* universe that discovers this, then there continues to be only *one* that will do the next experiment... at some point something is sent far enough back that it will change the actual discovery of the mechanism and the discovery ends there. The main thing is, no one is the wiser until something doesn't come back, and then you start to wonder just what is going on and *why* the longer period things just aren't working out... that has got to be frustrating, it was all going so well!
A fourth view is the H. Beam Piper Major and Minor levels view with only one race (Level 1) knowing that all other parallel time series exist. In this you travel side-ways in time across different levels determined by Major events and Minor sub-outcomes within a Major event framework. This only gains the appearance of going backwards in time if you find another timeline in the same Major event framework that is just slightly behind yours in historical events and time. Note that it isn't a 'duplicate' but a cascading series of events to formulate a timeline similar to how yours was in the past. That is unlikely, but has its own genre of fiction... still it does nothing for the timeline you are in.
A fifth view is the 'by gosh every possible universe is out there' view. Here every single possible outcome to every event does, indeed, exist. Even more fun is that each of these universes exists for exactly one period of Planck time (5.39 E -44 s .... yes that is a decimal followed by 43 zeroes before getting to the actual digits for your fraction of a second). There are some observations challenging Planck time as the smallest division of time possible (which would also start to re-write the Standard Model of the Universe). Basically every second has such a huge number of universes encompassing it and consciousness becomes a 'persistence of vision' idea traced through those contiguous universes with high enough similarity to allow them to be strung together as a construct you can be conscious of experiencing. So long as there is a high enough degree of similarity between each succeeding universe, you can have lots of things going on, and once a change is introduced all cascading experiences must include that set of changes. There is only causality within the Planck time universes, not along the consciously experienced path (although the laws of physics do wind up with very similar outcomes for that larger set of experiences). It is the 'cheap and easy out' to make slash fiction: just add the time paths up and create the universe and there it is! Great for lazy writers...
Now what is interesting is that if you take the third view, then the original universe that sent the Terminator and Kyle back saw no changes, while the one they are in *did*. That future then sent two Terminators back and saw *no changes* but the one they occupied in the film *did*. By T3 you are getting into yet another universe trying to change the past and seeing *no changes* and absolutely unaware that this would happen as the predominance of the technology was still leading up to similar events. Even worse is that by knowing time travel is 'possible' and even witnessing discrepancies in events, each would assume that *it* was the originator of the changes even when that is not the case. What is happening is that a set of lower and lower probability futures is playing out getting closer and closer to the discovery of time travel and each time it seems to *work* and yet for the universe originating the change it *doesn't*. Painful, very painful...
We see this showing up in the changing age and birthday of Sarah Connor: each timeline is undergoing a change that is removing the potential future that made it possible and altering, in some way beyond the film, the universe that is the recipient of the change. By T3 the actual birth year of Sarah Connor is shifted by up to 6 years, while it was only shifted by a couple of years in T2. Now if you are in a universe that has *that* as its basis and start sending stuff back into time more or less regularly, you will finally get a cascading set of changes in which no configuration of events will yield that future: it has self terminated.
That may be trying to send something further back, say back into the 1950's, to try and mess with events, and that derivative universe, not even well aware that time has varied so much, decides to try it *again*. At some point this tips into a cascading position that doesn't yield that capability by that general set of forces and the forces themselves no longer appear in history. They have screwed up the past so much that the next change yields something different. That change may still have a *last gasp* attempt to re-do history, although they will be unaware of that from their future perspective. They will never know that it didn't work as they will be gone as a potential in anything close to that configuration and a new universe will appear with only their artifacts remaining.
That is this story.
A time out of place.