It looks like Hermione forgot her Time-Turner in our fanart Harry Potter Time-Turner Sticker. It is a magical device used for time travel. It is a unique timepiece resembling an hourglass on a necklace. The British Ministry of Magic encased an Hour-Reversal Charm in the time turners they created for additional stability. The number of times one turned the hourglass corresponded to the hours one traveled back in time.