What do you do if you can travel to your past and change what you did? One thing Tim does is travel back from his father's funeral, and several times thereafter, to when his father was still alive. The first time he does it, he comments that he has plenty of time. Yet the way time travel works in this film, that should set off our alarms.
In the beginning of the film, when Tim has discovered that his Dad is not playing a terrible joke on him, he asks what to do with the ability. Dad says that for him it was always about reading, and we imagine at that moment that he lives through a day, then goes back to that morning and relives the day simply reading whatever his present book is. Yet that would be a recipe for disaster: he would be undoing whatever he had done.
The solution for Dad is simple: each day when he rises he announces that he plans to take the day off, calls out of work, spends the day reading, comes to the end of the day and before going to bed goes back to the morning, gets up and undoes his day off by doing whatever he ought to have done. Since he retains the memories of the erased history, he remembers his reading; since he has undone the lazy day off he can now go to work or do whatever is expected, with no one the wiser concerning his reading habit.
Unfortunately for Tim, it does not work quite that way for him. He has left the funeral, and certainly whenever he returns it will be the same moment he left--but in doing so he has disrupted whatever he was doing at that moment in the past. If he was at work, or in court, or home with Mary, or watching Posy, or whatever else he might have been doing, he is not doing that, preventing himself from keeping his obligations, such that in the new version of history those tasks were never done. There are two plausible solutions, neither of them promising.
Tim could have set aside hours of time when he did nothing, or at least made note of those times when he was doing nothing he could not erase with impunity, and then targeted those times for visiting his father. There is some foolishness to this--if he and his father were both available, he could as easily have spent that time with his father as returned to it. He could, perhaps, have erased times his younger self spent with his father and replaced them with times spent when older, but it is not clear that there is an advantage to this. In any case, there is no evidence that Tim was ever organized enough about his time that he could do this.
He could, then, do something like what we proposed for his father's reading times. He could go back, destroy his day completely by spending it with his father, then before returning to the future return to the beginning of that time and relive the part of the day he had erased. He would then remember those added times with his father, and not completely disrupt his life in the process. Of course, his father would not remember them--the times Tim spent with him would be erased. Perhaps that is not important; perhaps what matters is not that Dad spends that time with Tim but that Tim can spend it with Dad. Yet unfortunately we know this is not what he is doing, either. In his last visit to his father, his father recognizes that it will be the last visit of his son from the future--and that means that his father is aware of the previous visits, and thus that Tim did not erase them.
Perhaps, though, there is a third possibility. Twice in the film we see two people go back together--once Tim taking Kat, again at the end when he goes back to his childhood with his father. Perhaps when Tim spent hours with his father, they went back to the beginning of the time together, each preserving the memories of that time together, each then doing what he should have been doing with that time. It is a workable solution, and one they might have recognized; it simply is not suggested in the film.
We are not quite finished, however.