I think I found a bug for you. When I use the date selector to select the expiration date for a Coupon and choose a date after the start date (today), it returns an error that the date is before the start date.
Also, the date selector is vertically too tall and wraps under the page header, so it's difficult to select the month.
I was able to manually tab into the field (couldn't click to select it) in order to enter an expiration date manually and then manually reformat the start date to use the same MM-DD-YYYY format (which was not the default), then it worked.
I would recommend adding the action to launch the date selector from an icon next to the date field and allow clicking on the date field directly without launching the date selector.