Unique date field with formatter

Hi, I would like to make my date-field unique, but I'm using set- and get-formatter (server-side) with the editor.

Client-Side:

// ... 
editor = new $.fn.dataTable.Editor( { 
    ajax: "scripts/editor.php", 
    table: "#holidays_overview", 
    fields: [{ 
            label: "Datum", 
            name: "appointments.day", 
            type: "date",
            opts: {
                language: "de",
                format: "dd.mm.yyyy",
                showOn: "focus",
                todayHighlight: "true"
            }
        },{
            label: "Feiertag",
            name: "appointments.event"
        }
    ],
    i18n: DTE_i18n
// ...
});

Server-Side:

// ...
Field::inst( 'appointments.day' )
->validator( Validate::notEmpty( ValidateOptions::inst()
    ->message( 'Bitte geben Sie ein Datum ein.' ) )
)
->getFormatter( Format::dateSqlToFormat( 'd.m.Y' ) )
->setFormatter( function( $val, $data, $opts ) {
    return date( 'Y-m-d', strtotime( $val ));
} )
->validator( Validate::unique( ValidateOptions::inst()
    ->message( 'Das Datum ist bereits vergeben.' ) )
)
// ...

But currently I can still create more entries with the same date.
I guess it's because the unique-method compares the database value in format "YYYY-MM-DD" and client-side value in format "DD.MM.YYYY".

Is it possible to set the formatted values to unique? Like include an formatter into the unique method?
Thank you in advance.