Stephen Hawking describes that it is impossible to travel into the past using a very persuasive feedback-effect phenomenon.
In the end, I think a wormhole like this one can't exist. And the reason for that is feedback. If you've ever been to a rock gig, you'll probably recognise this screeching noise. It's feedback. What causes it is simple. Sound enters the microphone. It's transmitted along the wires, made louder by the amplifier, and comes out at the speakers. But if too much of the sound from the speakers goes back into the mic it goes around and around in a loop getting louder each time. If no one stops it, feedback can destroy the sound system.
The same thing will happen with a wormhole, only with radiation instead of sound. As soon as the wormhole expands, natural radiation will enter it, and end up in a loop. The feedback will become so strong it destroys the wormhole. So although tiny wormholes do exist, and it may be possible to inflate one some day, it won't last long enough to be of use as a time machine. That's the real reason no one could come back in time to my party.
I have no idea what I'm talking about but, it should be possible to travel back in time, somewhere else. Somewhere very far away, as long as the wormhole only existed for a short amount of time. This is because the radiation, which travels at the speed of light, would never reach the beginning of the wormhole in time before it closed, therefore eliminating the feedback phenomenon possibility.
So.... If this is true (which I'm sure its not for some reason which is painfully obvious to a physicist). That means that you could go back in time somewhere else, and then go back in time again, but this time back here at Earth. Of course you'd be way far in the past. Then all you'd need to do is go forward in time from that point on to get to the present, or any point in the past.