The possibility of time travel within the Harry Potter universe may seem to allow many plot holes, but characters appear to use them for trivial tasks that have no effect on existence as a whole. The one notable use of a Time-Turner within canon, the Rescue of Sirius Black and Buckbeak, obeys the Novikov Self-Consistency Principle. This theory of time-travel, stating that "Nothing can be changed because anything a traveler does merely produces the circumstances they had noted before travelling," is incidentally reminiscent of J. K. Rowling's employment of self fulfilling prophecy, but while prophecies within canon are relevant only to the degree that characters place relevance on them, as the books state Harry Potter and Tom Riddle could walk away and void the prophecy if they chose to, the same cannot be said for the series if any major events within the books or the Harry Potter universe as a whole have been related to the Time-Turner's misuse.
It is also a possibility that it is impossible to change the past, leaving a Time-Turner to only let users see what actually happens at another place. Examples that can explain this is that in the actual time, Harry was also hit by a snail shell, and Hermione still heard the crack of a branch. Which was what happened in the past.
The Time-Turner is one of the many collectables offered by the Noble Collection.
Part of the inscription on Hermione's Time-Turner - "nor have I yet outrun the Sun" - seems to suggest that Time-Turners can only travel backwards in time, not to the future. Alternatively, this may suggest a limit on how far back the Time-Turner may travel. It is possible that Time-Turners can only travel as far back as the beginning of the day.