A couple of years ago GrowthHacker.tv came up on my radar.
They deployed this hourly countdown to all their visitors. At the time, it actually got me over the line to buy.
Unfortunately, there isn't any logic that stores the visitors response so it just keeps showing it to them.
Using Bento you can deploy this same strategy (code here https://codepen.io/bentolibrary/pen/XEaoER) but only show it to specific users. This'll make sure your audience doesn't feel like they're in a gimmick.