The value that you see there is not the date, but the ID of the referenced entity.
Mendix stores date and date and time, if the attribute is made localised it stores as per the users time zone.
else it will be as per the server time
If you are using Mendix domain model with Mendix UI, the form elements that gets passed to any Microflow from a page button, will already have those values converted to appropriate data type. I dont understand why do you want to convert this explicitly yourself.
You can use parseDateTime('2015-05-21', 'yyyy-MM-dd')