A timer that goes back and forth.
There's probably a real name for this, but I don't know what that is.
When you click the screen, the timer starts going in the negative direction. If you tap it again, it starts going in the positive direction.
You can pause and unpause the timer.
This is just an implementation detail, so if you just want to use the product, you can safely ignore the following.
However, if you wish to contribute, take note of the following footgun: this codebase makes use of +0 and -0. You have been warned.