I think this question was pulled as a duplicate when it shouldn't have:
While the question is about Time-Turners, which seems to be well-documented and discussed, this question has not been asked, nor answered, as far as I can tell. The question which it is being linked to as a duplicate:
Specifically does not answer the question. It appears to me that @Richard pulled the plug way too quickly on this question. The answer given by @alexwlchan has a great explanation, which is not a duplicate of another answer as far as I can tell.
IMHO, this question should be taken off the duplicate list and revitalized.