I have a query that is used to display search results from search.  The query is in search.php. It shows custom posts and pages in the search result, which is what I want. I have a custom post type called 'event', each post has a date stored in `_event_start_date`. I want to compare `_event_start_date` with the current date and exclude event posts that are in the past from the search result.

My working query:
 
    if ( have_posts() ) : ?>
    <h1 class="page-title"><?php printf( __( 'Search Results for: %s', 'shape' ), '<span>' . get_search_query() . '</span>' ); ?></h1>
    <?php while ( have_posts() ) : the_post(); ?>
     <a href="<?php echo get_permalink(  ); ?>"><?php echo the_title(); ?></a>
     <?php endwhile; ?>
     <?php else : ?>
     <p> there were no results </p>
     <?php endif; ?>

How can I include `_event_start_date` in the above code, so that if an event has already started it is not shown in the search result?