Riley aka Helicopter Girl:
"I started working at the power plant in 2008 as Daniel Carey's secretary, a year before everything happened."
Then Jenny would be 31 years old by the time of the accident not 38 like someone else said. So if she went back to 1989 and stayed there she would be 61 "now" like Jackie, she just have to change her name in the past like Jack did. And if i remember correctly Skylar butt messages seemed to imply she was in 1909 so apparently the machine works in 10 years intervals, like i said i'm 99% sure Jenny is Jackie.