I don’t understand the Evergreen and Restart Countdown

In the doc, those feature are not clear.
Please, can you explain how the evergreen feature works, and how the restart countdown works?

I need to have a timer going down from 24 hours, and reset when it reaches 0.
And I need this timer to be consistant with each page reload.
But I can’t modify the dates every single day.

So, does evergreen and timer reset help me with this?

Thanks,