No Proper Lady by Isabel Cooper
My rating: 3 of 5 stars
The apocalyptic world in which Joan resides is in dire straights of a savior. As the final hope to a better life, Joan finds herself entering the stone circle. The mission: Go back in time and destroy the trigger to her world's destruction. Joan is transported to 200 years in her past. Along the way, Joan befriend a wizard and his sister, whom ultimately assist her in her mission.
I agree with other reviews, stating this is very "Terminator" like; substitute robots and human destruction with witchcraft and demon hunting.