I believe the way it's supposed to work is that there isn't just a single time-line. Each time something is changed the time-line splits into a new line. So the fact that Skynet basically creates itself is redundant because in the new time-line created when the Terminator was originally sent back in time Skynet exists so the time-line in which Skynet does not exist (or rather it does not exist for many more years) is no longer relevant.
The thing you need to remember is that when the Kyle tells Sarah about judgement day he is referring to his own timeline and not the parent timeline in which it may not have occured for many more years.
There is also the possibility that I'm talking complete bollocks.
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