Plugin expects the date in this format:
image

However, when you use the date picker the date format changes to this:
image2

Resulting in this when trying to save…
image3

Flycart, the Woo Discount Rules plugin creator, have investigated this and found that the theme date picker is overriding the plugin date picker resulting in the error. This only started happening in the last 2 weeks when the theme was updated.

Client has insisted on doing it on staging which makes sense, please help 🙂