I have not, nor will read Cursed Child, but I dont think many people count is as definitive canon anyway.

For a long time I thought time turners operated as closed loops, that to turn back time you cannot change that which caused you to want to turn back time. This made sense to me and prevented time turners from being unbelievably stupid, not to mention the seeming confirmation that we get of this with the events of Prisoner of Azkaban.

However, I was re-reading the book and came across a few passages that imply that this isnt the case.

These passages to me imply that it is possible to change time, but it is incredibly dangerous and often leads to awful results. The way that both Dumbledore and Hermione talk, using the time turner to change the past is incredibly illegal, but not something impossible. Otherwise, why would you need to make it illegal after all.

But if this is the case, why is Harry's use of it a closed loop wherein he saves his past self because he knew he had already done it?

 

While the "law" here might apply to an actual law by the Ministry, there are also the laws of time itself, if you take Pottermore seriously.

https://www.wizardingworld.com/writing-by-jk-rowling/time-turner

So you can breach time's "laws" too.

This also gives an example of the disastrous consequences of not following the rules/laws (making sure it's a closed loop). 25 people are "un-born", and Tuesday lasts two whole days while Thursday lasts only four hours.

So it's illegal to actually change the past due to stuff like that. Though I think Hermione here thought they really would change the past, not just create a closed time loop. Either that or one of the wizarding laws on time is to never use it for anything other than trivial time management. But given she says "Nobody is supposed to change time", I think it's the former.

Might be wrong or forgetting something here.

 

How can you even know if people are 'unborn' at all though. I mean if they were never born they never existed, so how did anyone figure out that they would have existed (and exactly 25 of them)? Time Turner's are probably a huge plothole, no matter how they work, which is why Rowling got rid of them. However my own interpretation of them is that, though you cannot 'change the past' with them, their mere existence creates an alternate timeline of sorts, usually beneficial to their wearer. Harry and Hermione did not travel back in time to save Sirius in Book 3, because he never died, but if Hermione hadn't received the time turner at the beggining of the year he probably would have. Likewise Harry could not have taken a time turner to save Sirius in Book 5, but maybe if he had gotten one earlier maybe the event in question would never have happened. I suppose my way of thinking is that the time loops are already incorporated into the 'general' timeline, so you don't do anything but follow said timeline when you put yourself in one.

 

You can definitely change time; it's just set up to look like you can't. The Ministry almost certainly created all of the time turners, so during the design phase they came up with the idea of making everything look like a closed loop. If they were going to pass laws to prevent people from doing more than going about their day a little differently, it makes sense to keep people from getting ideas. The result is a process the film actually did pretty well, where it looks like everything had already been done, but the time travelers had to go through the motions.

 

My knee-jerk answer is no, you can't. The actions you undertake while back in time were always part of the sequence of events. Regarding the three quotes you mentioned:

Number one can be interpreted not as Dumbledore mentioning it because grave time-related consequences would occur if they were, but legal ones. What is at stake is Buckbeak's life, Sirius' soul, as well as potentially their own lives/souls. Using the Time Turner this way is majorly illegal, for obvious reasons.

Number two could also be just general over-protective legislation trying to prevent something like a Grindelwald sympathizer trying to ensure he wins vs Dumbledore (the Wizarding World equivalent of killing Hitler).

Number three could also be interpreted as using some other method of time travel. Like, Time Turners are one of the safest methods around (relatively so, anyway) of time travel, created after lots of experimentation in the Department of Mysteries. That's probably why Hermione, a fourteen-year-old girl, was allowed to use one--hell, it might have been loaned out as part of a test to see what sort of consequences long-term use of the Time Turner has. Earlier versions of time travel might not have safeties, like if the time-sand had a tendency of spilling out and accidentally de-aging people out of existence. (De-aging out of existence is my theory of what being 'un-born' means.)



That being said, there's room for both the theory of singular unchanging timelines as well as potential catastrophic changes within the same setting.

For what the Time Turner was designed to do--go back in single-hour increments, no more than a couple hours at a time, make small behind-the-scenes changes or other in the background things like take extra classes--time operates on a closed loop. As long as the changes you make are minuscule, the effects they have on time are likewise tiny. Time operates as if there were just suddenly two of you operating independently for a couple hours, then goes back to one. In terms of "time is a river" analogy, this kind of usage is similar to dropping a small rock into it--the water parts around it, but continues heading in the same direction as before. Or in another way, the hole you poke in the time-space continuum is so small that it heals instantly.

Larger changes though, have similarly bigger effects. In this case, you are effectively dropping a boulder into the path of the river. Is the river dammed up now? Will it go along a different path? Will the rush of water affect things on the shore? The effects are unclear, and the bigger the "boulder" the bigger the consequences.

 

Absent the use of a time turner, when Hermione attended one of her 3rd year classes, she was missing one of the others that she signed up for.

Isn’t the use of a time turner in that context changing the past, even if in a small way?

 

Based on how Time-Turners and time-travel work in PoA, it is impossible to change time or - contrary to what Hermione claims - to kill your past self, since there is only one timeline. The only way those passages make sense to me is that Dumbledore was simply saying it was illegal for a looping person to be recognized to be looping, and that Hermione, despite having been using one for a year, doesn't completely understand how a Time-Turner works.

There seems to be strong predestinative element when it comes to Time-Turners in PoA, as in a loop just being suddenly created without Harry being aware of it in any way or having anything to do with it before he is saved from the Dementors by an older version of himself and is destined to then go through the steps of the loop before he exits it. It is all very absurd, even within a fantasy story, and 'all the Time-Turners being destroyed' in book 5 doesn't rectify it.

 

Perhaps not, since there was never actually a sequence of events wherein Hermione missed a class, because there were three of her at a time. That's always how it went down. Hermione time traveled to conform to what happened, not to change it.