QIf John and Kate KNEW the new date and time of "Judgment Day," why wasn't the T-850 sent back earlier, to give them more time to prevent it?
AFirst of all, it is stated time and time again that "Judgment Day" is inevitable. The T-850 mentions it several times. It was prevented once, but it can only be postponed. The human capacity to destroy itself is simply too strong, and an apocalyptic event was bound to happen sometime. The future resistance never had any desire to prevent Judgment Day, merely to protect the key players (i.e. John Connor and the future Kate Connor), while Skynet knew that wiping out their existence could tip the balance in favor of the machines. While Skynet wants to rewrite history in a desperate attempt to win the war, the humans are ultimately victorious and they merely want to preserve this timeline. Another reason is that John Connor exists solely on the fact that the war with the machines takes place. Kyle Reese is sent back, John is conceived. This would have not been possible without the war and would have created a paradox. John's fate and that of the war are intricately tied together, and has actually become part of the Terminator mythology.
AJohn Connor was 10 in T2, and at the beginning of the film, his mother Sarah is in a mental institution and he is with foster parents. He tells the T-800, "It was kind of like, 'Hey kid, your mom's a psycho; didn't you know?' It's like everything I was brought up to believe was made of bullshit. I *hated* her for that. But everything she said was true. And nobody believed her. Not even me." So at first he was just a careless kid because he didn't think he had anything to worry about. As the film progesses and he sees how all of it is true, he accepts it because he has both a Terminator to protect him and a mother to guide him. By the 3rd film, his mother is dead, and the Terminator had destroyed itself, leaving him all alone. While he thought they stopped Judgment Day, there was still the lingering paranoia that he hadn't. Now imagine not being responsible for two lives, but several billion. As John says in the opening narration: "I feel the weight of the future bearing down on me...a future I don't want." They expect him to lead and to win. He has become older, and the full extent of what is expected of him has also become clear. Every young person who is coming of age typically experiences some fear of the unknown future, and the fear of becoming responsible for the lives of loved ones. John needs to lead the remainder of mankind toward a seemingly impossible victory against a superior enemy. It is not hard to see how this causes him to get cold feet. Also, in order for him to become this great leader, three billion people have to die first, which adds further to his reluctance.John lives in a mixed state of fear and denial. In the opening narration, he says "The future has not been written. There is no fate but what we make for ourselves. I wish I could believe that...we stopped Judgment Day. I should feel safe, but I don't." On the one hand, he seems fully aware of things to come and his role in this, not believing Judgment Day was prevented in T2. But when he is confronted with the evidence for his suspicion, namely the appearance of another Terminator, his reaction is "You shouldn't exist. We took out Cyberdyne over ten years ago. We STOPPED Judgment Day!" During the entire film, he makes every effort to prevent Judgment Day, ignoring the Terminator's repeated claims that "Judgment Day is inevitable," instead of preparing himself for the worst. John is clearly not yet ready for the truth and not yet up to the task. Therefore this change in John's character is not a break in character. John's struggle to accept his future role as savior of mankind was made an important part of the story. In the end, when it is clear that the future indeed can't be changed, John symbolically accepts his destiny when the voice on the radio asks who's in charge, and he finally has the strength to say "I am."