The way I understood it is inside the box time flows backwards, so if you crawl into the box and wait a day when you come out of the box you'll be a day yesterday instead of a day tomorrow.

Of course, you can't go back to before when the box was created, but once you have made the box, you will always be able to come out of the box at the moment it was turned on.

Imagine if you had to tell a story, but all at once.

Also time travelling gives you cancer.

The box creates a loop between the time the machine is turned on and the time it’s turned off. Think of it like stretching a rubber band vs. normal time as stretching a cord. The characters would enter at the time the device was turned off and exit at the time it was turned on.