$meta_boxes[] = array(
    'id'          => 'ce_info',
    'title'       => __('CE info', 'comsi-ce'),
    'post_types'  => 'comsi_ce',
    'context'     => 'normal',
    'priority'    => 'high',
    'fields'      => array(
        // this works
        array(
            'name'            => __('Recurring start date', 'comsi-ce'),
            'id'              => 'comsi_start_date',
            'type'            => 'date',
            'inline'          => false,
            'timestamp'       => false,
            'size'            => '14',
            'save_format'     => 'Y-m-d',
            'js_options'      => array(
                'dateFormat'      => 'dd-mm-yy',
                'showButtonPanel' => false,
            ),
        ),
        // this does not work
        // Warning: DateTime::createFromFormat() expects parameter 2 to be string, array given in /Users/ronald/Sites/ronald.local:5757/wp-content/plugins/comsi-ce/vendor/meta-box/meta-box/inc/fields/datetime.php on line 175
        array(
            'name'              => __('Recurring dates', 'comsi-ce'),
            'id'                => 'comsi_dates_recurring',
            'type'              => 'date',
            'clone'             => true, // << this is the problem
            'clone_as_multiple' => true,
            'inline'            => false,
            'timestamp'         => false,
            'size'              => '14',
            'save_format'       => 'Y-m-d',
            'js_options'        => array(
                'dateFormat'        => 'dd-mm-yy',
                'showButtonPanel'   => false,
            ),
        ),
    )
);