The Novikov Self-Consistency Principle: wiki/Novikov_self-consistency_principle : is a model of time travel that assumes that time travel never generates paradoxes.
What you did in the past already happened.
This story explores usages of an unrestricted form of this model of time travel. It starts with the simple stuff and gets more complex towards the end. A number of techniques rely on the absolute requirement that paradoxes are impossible.
The late morning was quiet. A short line of people were queuing up in front of the teller at the bank, waiting for their turn. Outside, the lunch crowd was just starting to file into the street. None noticed Anne appear silently out of thin air in one corner, next to herself sitting in a seat with a laptop balanced in her lap. She quickly walked out of the bank branch and the curious little girl, whose boredom was suddenly replaced with confusion, approached the Anne sitting in the corner.
Anne looked up from her laptop as the kindergarten age girl tugged on the sleeve of her high school uniform. Anne smiled and bent down to listen. "Yes?"
"Don't you have to go to school?" the girl asked.
Anne winced internally. This little kid... "Yes, I do. I am in school," she replied, careful to keep her irritation out of her voice. Let her chew on that!
The girl looked bemused. Anne hadn't actually lied, however. She was still attending class. She glanced down at her watch. It was almost time.
Looking quickly around the bank branch, confirming that no one was watching, she leaned down to whisper conspiratorially, "When it starts, stay with your mother. "
The kid wandered back to her mother in the line with some awkward questions, and when the robbers came running into the bank branch, waving guns around, she quickly forgot about Anne. The police turned up less than a minute later. The totally surprised robbers hadn't gotten further than announcing that they were taking everyone hostage when they were rudely brought to ground.
Anne got up, closing her laptop. She took a step forward, gave a parting wink to the girl and simply vanished into thin air as silently as she appeared.
It was only later, in the after action report, that the police realized they had received a call for help at the branch nearly ten minutes before the robbery actually started.
Anne twirled her pen, listening to the math teacher with half an ear. The police sirens sounded outside and the one single gunshot went completely unnoticed by the class. While she would be on her way back from the bank now, Anne was also currently stuck in class.
Math was just a little boring after that particular adventure. Far too many people fell victim to the temptation to use their powers for crime and the police were only just starting to adjust their tactics to the upsurge.
The existence of superpowers was only just starting to come into view. The first person with them had appeared only a few years ago, and since their discovery, more and more people had come forward with sudden new powers that demonstrated little pattern in how they worked or their strength. The superpowers had resisted scientific explanations so far but there were many theories.
Of course, one of the more prominent people with superpowers, a telekineticist by the name of Joey, was actually serving as an advisor to the city governing board and assisting the police with more difficult cases, when he wasn't helping research into superpowers. Anne had thought that Joey was a bit crazy to spend his time fighting crime, in a manner of speaking. But she had to admit, as the police van carrying the thieves drove past the school, that doing precisely the thing she thought was crazy could be pretty fun. All the excitement, and none of the risk. After all, with time travel, what risk could there be?
The teacher started explaining the use of a new kind of graph and Anne closed her textbook. She could just teach herself later if she needed to learn it. Not like getting full marks on any exam, even a surprise test, would be all that difficult.
She looked down at the pad of paper in front of her that she was supposed to fill with notes. Hmm, I wonder how far I'd have to jump if I wanted to bring back the marked test paper. If they had a test tomorrow, even with all the questions and answers, she'd need at least one night to study. And give the teacher three days or so to mark and return it. That would mean that she'd get a visit from herself from four to six days in the future, taking into account the weekend.
Anne didn't even know if she could jump that far. Besides, even if she could, memorizing an entire test worth of answers would be about the same effort as actually studying for it. Especially since her future self could always teach her and would understand exactly how do so.
After all, that was how she learnt to time travel just this morning.