Reff: Lucy (2014) : ExplainedLucy (2014) : Explained
I don't think she's traveling through time.
Once all the CPH4 goes in, her brain nears 90% utilization. As Lucy's brain function elevates further and further, she is able to tap into and use all forms of energy around her. She can look into the past or the stars and galaxy because they are all forms of light energy that she can access. (eg: The reflection of yourself in a mirror is you looking at the past because light takes finite time to travel from you to the mirror and back to your eyes)