I beg to differ,
ariels: the only model that makes sense (If you allow time travel at all) is one with an infinite number of
parallel universes.
In that model, whatever a time traveller does just creates a new universe, but doesn't change the old one that the time traveller came from - thus, time travel paradoxes are not a problem (I'm not absolutely sure if this holds true with multiple time travellers and crossover manipulations).
Another one that makes a bit of sense is one in which time travellers are freak events that create loops which perpetuate themselves: in Terminator, the time traveller doesn't actually change anything.
Any other model has at least one major logical flaw: if there is a single timeline which a time traveller can change (the by far most frequent model, because it allows for cheap let's go back in time and prevent <insert negative event> from happening plots), it is a total contradiction, because change and causality is a function of time - but you just fixed time inside the timeline you are observing! Time progresses along that line, any kind of change happens along that line!
This is the root of time travel paradoxes. It might be resolvable by using some kind of "meta-time", but certainly not by using conventional temporal and causal thinking when talking about "sequences" of evens that don't follow the kind of order which we live in. That way of thinking results in glaring logical flaws like an alteration caused by a time traveller manifesting as a gradual change decades later that leaves people enough "time" to react in some way (see Back to the Future or Tenchi Muyo in Love for examples).