Daniel Faraday said that time is like a string. You can move forward or backward along that string, but you cannot create a new one. "What happened, happened," he stated in "La Fleur." Despite what others have theorized, no one can alter the course of that string once it has been set. The Losties may move back on that string, but they can't change where it's going.
Everyone operates under semi-normal space time realities in the show. The Island itself may have bent space and time slightly within a radius, thus accounting for the special procedure needed to "find" it from the outside world and other anomalies like the rocket test Faraday tried or the loss in time between the boat's doctor washing up on shore. But barring these explainable phenomenon, which we can attribute the special nature of the Island itself, everyone on and off the Island experiences time as you or I would in the real world (though Richard Alpert and Desmond may prove the exceptions to my theory).
When Ben Linus moved the frozen wheel, he moved the Island, himself, and the Losties on that string in different directions. Something happened when he moved it that resulted in the wheel malfunctioning, thus giving way to "the record skipping." The Losties were then moved along that string backward and forward many times, until John Locke moved the wheel again, therefore fixing the skipping. Several, if not all, of the Losties, including Sawyer, Juliet, Miles, and Faraday, were then left on the string, albeit not at a point they were comfortable with.
Once the direction of the string, the future, has been laid out, it is impossible to deviate from its course. Any and everything we know has happened, has happened. Even though the Losties are stuck back in 1977 with DHARMA, they cannot affect what will happen in the already determined future. They can't stop the Purge from happening, Flight 815 from crashing, or Widmore from sending Keamy and the mercenaries to the Island. Any theories suggesting anyone can change anything are simply wrong.
As to what hasn't yet happened, we cannot say. We don't know how far along that string has developed. Perhaps it's already laid out for eternity, perhaps there's an end to it where the living are actively building its new direction. We don't know about that yet and may never. What can be dismissed from what we do know is that the characters can move backward and forward, but never onto a new string.