Time is Everything

In an alternate universe where myths are reality, a young girl discovers that the rumor about the clock on the rooftop of her school is true, and that said clock is no ordinary one. Have you ever said or done anything that you later think back to, regretting?

Of course you have! Silly me, you're a human. If you stand under the clock and say where you wish to go in time, you'll be given a stop watch. However, in return, the clock will take the thing you the most value the most.

Now... I wonder who that is?