It's really not all that hard to figure. String theory---and strangely enough, the Bible---offer answers to this. Briefly, the universe is made up of not FOUR dimensions but TEN dimensions. Four we see---length, width, height, and time. The other six are hidden from us.
Now, add in the spiritual planes of Heaven and Hell---both having the three dimensions of length, width, and height, but both are eternal and thus lacking the fourth element of time. Now add 'em up---4 (earth) + 3 (heaven) + 3 (hell) = 10. String theory confirmed! What this means is that as the physical and spiritual realms are linked, linear time and NON-time are similarly linked.
Think of each point along the linear timeline being connected to one SINGLE point of non-time---an eternal "now", as I call it, where every point of linear time (past, present, future) is represented in the immediate present in non-time. As Jesus put it, speaking to the scribes and Pharisees---"Before Abraham was (past tense), I am (present tense)."
As non-time is connected to every point in linear time, it should be possible to move from your current placement in linear time to non-time, then from non-time into any point in linear time---regardless of where along linear time the traveller had originally come from. This would explain Livia moving forward, and Dan moving back. This doesn't explain ALL of it---like how Dan and Livia return to their NATIVE time---but it's a start.
Sorry... showing off my inner geek
