<?php

function SearchFilter($query) {
    if ($query->is_search) {
        $query->set('meta_key','_event_start_date');
        $query->set('meta_query', array(
                      array('key'=>'_event_start_date'),
                      array('key' => '_event_end_date', 'value' => date('Y-m-d'), 'compare' => '>=', 'type' => 'DATE')
                    ));
    }
    return $query;
}
add_filter('pre_get_posts','SearchFilter');