Following on from my previous reply, I've got two different Portals, both at 10.7.1 and both have default dates set to Great Britain, dd/mm/yyyy, in organisational settings.
If I create a blank dashboard, add a date selector and set to date picker, one portal displays it as dd/mm/yyyy and the other displays as mm/dd/yyyy. In both cases I'm logged in as an admin user and viewing within the same browser session in IE11. Unfortunately the one that is showing incorrectly is our production Portal so it's not necessarily fixed in 10.7.1 and there might be another factor involved here. Have checked date/time and regional settings on both servers and they're exactly the same.