Yuriy

register_setting('my_option_group', 'my_option',array($this,'my_validate_options'));

array( 
        
            'relation' => 'OR',
                    array(
                        'key' => 'event_datestamp',
                        'value' => array($date_start,$date_end),
                        'type'    => 'UNSIGNED', 
                        'compare' => 'BETWEEN'
                    ),
                    array(
                        'relation' => 'AND',
                            array(
                                'key' => 'event_datestamp',
                                'value' => $date_start,
                                'type'    => 'UNSIGNED',
                                'compare' => '<='
                            ),
                            array(
                                'key' => 'event_datestamp2',
                                'value' => $date_end,    
                                'type'    => 'UNSIGNED',
                                'compare' => '>='
                            )
                    )
                )