Watch of Time Control

John stepped outside his house to the place he last saw the slate. Instead of the slate he saw a watch.

John was confused, where did the slate go. But he didn't have time to figure out about that.

He picked up the watch from the ground and inspected it, hoping to find something cool in it.

He found a button on the watch, several in fact. All of them were engraved with options like, Stop, Resume, Rewind.

What was this thing?