JS date/calendar issue on Magento 2 - v2.2.3

There seems to be an issue with saving dates for things like coupons, promotions, special prices, etc. Also filtering the sales grid by date can crash the admin 'there has been an error processing your request'. if we add a date filter, go into an order and then come back out of it, it will say 'invalid date format' in the filter. Whenever we try to save a coupon with a specific date it fails, giving and error message like: "Something went wrong while saving the rule data. Please review the error log." or "invalid date format". The default storeview was created as a french storeview, with english, irish, australian, and american storeviews built ontop of it. We tried forcing all storeviews to use en_US, but it didn't solve the issue. We did install a free plugin from Magonex to change the default time format, and that did allow us to at least save promotions, etc, but we still have to blank out the date on promotions and special pricing whenever we want to save any changes and so cannot schedule a promotion or special price, etc. We also noticed once we started adding new plugins that came with a calendar date option that the Magonex plugin was not solving the issue for these. So we are looking for a complete 'root cause' fix for all date elements, and are happy to use a single date format if necessary, preferably the dd/mm/yy format. We are using Magento 2 - v2.2.3 We will give you access to a fresh dev site. We will need to know exactly how you fixed this, as we will need to replicate on our live site ourselves. Many thanks.