functions.php

<?php

/**
 * Add search weight to more recently published entries
 */
function my_searchwp_add_weight_to_newer_posts( $sql ) {
	global $wpdb;

	// Adjust how much weight is given to newer publish dates. There
	// is no science here as it correlates with the other weights
	// in your engine configuration, and the content of your site.
	// Experiment until results are returned as you want them to be.
	$modifier = 15;

	$sql .= " + ( 100 * EXP( ( 1 - ABS( ( UNIX_TIMESTAMP( {$wpdb->prefix}posts.post_date ) - UNIX_TIMESTAMP( NOW() ) ) / 86400 ) ) / 1 ) * {$modifier} )";

	return $sql;
}

add_filter( 'searchwp_weight_mods', 'my_searchwp_add_weight_to_newer_posts' );