Wobag Time is an illusion, lunchtime doubly so...
For your benfit, Jack was a human time agent from the 51st century, the Time Agency being some kind of CIA of time travellers. Jack quit the Agency when they deleted 2 years of his memory.
I missed the last 2 episodes before last nights one, so have no idea who the new American is on board the Tardis.
Jack was making a living as a time travelling con man when he tried to pull a fast one on the Doctor, believing him to be a time agent.
The Doc later picked up Jack from his space ship just prior to it being destroyed, and he has been on board since.