"The events take place in an alternate timeline from that of Terminator 3: The Rise of the Machines. Skynet was now implemented on April 19, 2011 and will begin its attack against humanity on April 21. The protagonists travel in time from 1999 to 2007 and take actions to prevent Skynet's activation." -Wikipedia
Just so you know.
We all know what happens next. Godspeed.