Impossible to import date fields to a multi record custom field set

When you import to a multi record custom field set containing a date field, these are rejected 100% with the message "value xxxx-xx-xx is not of the right field data type: Date" This occurs no matter what format you select on the import UI or what format the date field is in the input file, or what date format you specify in the custom field set. It appears to be impossible to import any date fields for multi record custom sets.