Time is Convoluted in the Land of Britain

Oleandra and Tracey's first class of the year was Divination. It was also Daphne's, ex aequo with Muggle Studies and Arithmancy. Now, the class schedule said that the Divination classroom was located in the North Tower. The problem was that the schedule didn't include a map, and none of them had ever been there.

"It's called the North Tower, so it stands to reason it should be on the north side?" said Tracey.

"Honestly, have any of you read Hogwarts: A History?" said Daphne, rolling her eyes. "The North Tower was named after Dudley North, a Hogwarts graduate."

"It really is on the north side," said a passing fourth-year student who had overheard them talking. "Just take the bridge from the Great Hall, it'll be right before the Herbology greenhouses."

Oleandra had never seen her sister blush so hard before. At any rate, the directions their upperclassman had given were correct, and after a short walk, they ended up in the North Tower without any issues.

"So, you're skipping classes?" Oleandra told her sister in mock disappointment. 

"What are you on about?" asked Daphne, confused.

"Well, if you're here, then that means you're not going to Arithmancy or Muggle Studies. You're skipping."

At that moment, a strange feeling washed over Oleandra. In her mindscape, the time rune Dagaz had begun spinning madly. The passage of time was still flowing ever forwards, but it had grown convoluted; it had begun looping in two separate places near her.

And yet, there were no paradoxes. Time in Oleandra's world was linear, and parallel worlds were parallel for a reason; it was impossible to jump from timeline to timeline by changing the past. Firstly, it was in fact nearly impossible to change the past; any attempts to do so would result in catastrophic changes as the universe corrected itself. Secondly, if one did manage to change the past, it would have been because you had always been meant to do so, which also meant that nothing would have actually changed overall.

1-The universe would resolve the grandfather paradox in the simplest way; by deleting everything involved that didn't make sense. If you went back in the past to kill your grandfather before your parents were born, everyone from you to your grandparent would simply cease to be. No perpetual loop, no paradox. Strangely enough, people would still remember the people who had stopped existing, a grim reminder from the world to stop messing with time, perhaps.

2-Judging by the existence of seers and prophecies, the concept of destiny definitely existed, but otherwise, the future was mostly still up in the air. Which meant it was still technically possible to change the future by altering the past. As long as one hadn't observed an event, it would remain in a strange state of superposition; all possibilities existing at once. Therefore, if one thought an event would occur, but hadn't seen it happen yet, there was still a chance to change the expected result by travelling to the past.

"You can't be serious, Sister," gasped Oleandra. "You're messing with time, just to fit in more classes in a day? How are you even doing this?"

"I haven't done anything yet," said Daphne, shrugging her shoulders. "That would be my future self, an hour from now when Divination class is over."

She paused for a second.

"Professor McGonagall made me swear not to tell anyone, but since you've guessed it, I'm no longer bound by my promise to not discuss it with people who don't know about it," Daphne continued. "The answer is, the Ministry's Time-Turners."

"Even I can tell this seems wildly irresponsible," noted Tracey. "Giving time-travelling magic to teenagers."

"You have no idea," said Daphne. "Professor Snape had to sign all sorts of letters to the Ministry of Magic so that I could have one. The request did go through, but I have to share my Time-Turner with Granger."

"They only had the one?" asked Oleandra.

Daphne shook her head.

"That's not it. It would be way too risky if two students of the same year had access to two different Time-Turners at the same time, so each grade gets a maximum of one," explained Daphne. "Besides, since we're taking the same classes, it made sense for us to share."

"There's a whole bunch of rules to memorize as well," she continued. "Rule number one, never let your past self see yourself."

"That would be freaky," admitted Tracey.

"I'm used to it," said Oleandra glibly, pointing at her sister. "I see myself every day. Look, there I am!"

"Not only could it lead to accidents," explained Daphne, "it would also mean that my past self would need to perfectly memorize my present self's words and actions, so that she could replicate them when it's her turn to go back in time. Also, I'd need to have already seen myself in the past to do that (which I'm already not supposed to do, but if it did happen, then I would absolutely have to do it, or else bad things would happen)."

"I'm confused," said Tracey, raising her hand as if she were in class.

Daphne had pretty much given up trying to dye her hair back to its former colour, so at least she didn't have to worry about confusing herself for her twin. In any case, Oleandra preferred to gather up her hair into a ponytail rather than leaving her long dark brown hair flowing freely, as it just got in the way when moving around in bursts.

By the time Daphne was halfway through her explanation, people had begun to arrive for their first Divination class, so the conversation had to be shelved for another time, as the Time-Turner was supposed to remain a secret.

"'Sibyll Trelawney, Divination teacher,'" Harry read on a plaque. "How're we supposed to get up there?"

There were no stairs to continue ascending up the tower, only a singular trapdoor in the ceiling. But as if on cue, it suddenly opened up, letting a ladder down. It was a good thing nobody at Hogwarts was too badly disabled, or the school might have been sued for accessibility issues.