Example can currently be seen at https://test.wikipedia.org/wiki/Special:Contributions
The hint text for this page is inconsistent within the page.
*When the page loads there is hint text to use "mm/dd/yy" format
*Shortly after load the hint changes to "No date selected"
*On click the hint changes to "YYYY-MM-DD"
*On selection the date is reformatted to "Dayofweek, D, LongMonth, YYYY"
*On click the hint changes to "YYYY-MM-DD"
*On selection the date is reformatted to "Dayofweek, D, LongMonth, YYYY"
I don't see this as a bug. It is indicating that it accepts inputs in the YYYY-MM-DD format, and then showing the date in a more human-friendly way.
Considering cancelling this as the symptoms have change a bit. The initial mm/dd/yy format is no longer displaying (which was confusing as that was not a valid input format); the YYYY-MM-DD format is also accepted for keyboard input.