The story of this manga sux a bit and is too unrealistic, I mean the author hasn't even thought about the physical laws of time and space and so on.
I mean the story of a girl messing in the past, if she changed sth ( and that can happen by making 1 step) she would change the future and her existence, meaning she would dissappear, never travel back in time and the circle goes on.
If u want to see a masterpiece about time traveling then watch the movie "Time Machine", it's a great movie and it would teach u many things about life and the order of things in this world.
Besides, basing on my thoughts time traveling, if possible, could theoretically work only into the future ( a future created after your time jump). ;P