Strtotime ACF datefield parses 1 jan ’70

I’m using this code but the output results in 01.01.70. Clearly my code is wrong. But for the life of me I don’t know where. The date comes from datefield ACF called ‘datum_event’.

     if ( function_exists( 'get_field' ) ) {
    $date = get_field( 'datum_event' );
    if ( $date && is_string( $date ) ) {
        $html .= '
' . date( 'd.m.y', strtotime( $date ) ) . '
    ';
    }
}

I was expecting to have the result of the input in the ACf datepicker.