When Harry and Hermione leave from the hospital wing using the Time Turner, Dumbledore says it is ten minutes until midnight, but on the way back to the ward after they rescue Sirius, they hear Dumbledore say to the other "them's" that it is five minutes to midnight. So unless Dumbledore used a Time-Turner to go five minutes into the future, I think we have a slip up... Time... It's a mysterious thing...