I can't leave this alone, here goes:
The best explanation I've gotten for 'time' is that it looks a tree, with new branches starting whenever an event happens. There has to be 2 (or more) seperate outcomes to this event that can alter the flow of time, yielding one branch for each event. An example for a time altering event would be Japans decision to bomb pearl harbour, they do and loose the war, or they don't and something else happens.
This concept makes travelling forwards in time much more difficult than travelling backwards. Travelling backwards is like climbing down a tree, but going forwards, you have to know exactly what branch to go to and how to get there.
Originally Spock did not want to get too involved since he wanted to keep the timeline intact. He's been in the past before and avoided altering the future, so he was able to return to his time (I don't know the original series too well). The Romulans attack on the USS Kelvin altered the timeline significantly, and Spock realized there was no going back, so he made the best of it.
Again, I'm not trying to turn this into '"I'm right you're wrong" I just think time travel is fun. Heres my primary reference:
http://www.youtube.com/watch?v=Jkxie...eature=related