Include post_date in search

I was able to receive the solution from the developer. Add the below code into functions.php and then rebuild the Relvanssi index. This worked for me.

function rlv_index_post_date( $content, $post ) {
    $date = get_the_time( "F Y", $post->ID );
    $content .= " $date";
    return $content;
}
add_filter( 'relevanssi_content_to_index', 'rlv_index_post_date', 10, 2 );