I hate to add another review that just says "it doesn't work," but... it really doesn't work.
I can see the new tab on the product details. I enable the schedule, and set start and end* dates. I save the product, and... nothing happens. The "Add to Cart" button is still visible. There's no countdown timer. Absolutely nothing is different.
But even if it worked, it still has problems:
1. It should not require me to enter start/end times using GMT. Wordpress already knows my timezone; why can't I enter the times in my own time zone and let the plugin figure out the difference?
*2. It should not be necessary to enter an end time! If I want to say, "Availability will begin on this date and continue henceforth," that should be valid.
3. And if it is necessary to enter an end time, IT SHOULD WARN ME IF I DON'T, instead of just silently not saving what I've entered!
4. The plugin description tab says there are shortcodes. Why are there shortcodes? What do they do? In what circumstance would I want to use one, and where would I put it?? So many unanswered questions.
5. There seems to be no support at all. None of the posts in the forum have been answered
WP 5.5.2 (the repository warned me that it was untested past 5.3.5, but usually plugins don't fail so catastrophically within just 2 minor versions).