Arbitrary SearchWP weight multiplier on date as Custom Field value

searchwp-customizations.php

<?php

// Modify SearchWP calculated relevance using multiplier.
// @link https://searchwp.com/documentation/knowledge-base/add-relevance-weight-date/
class My_SearchWP_Date_Modifier {
	private $post_type       = 'post';
	private $meta_key        = 'event_date';
	private $modifier_past   = 0.5;
	private $modifier_future = 1.5;
	private $alias           = 'myswpdm';

	function __construct() {
		global $wpdb;

		// Modify SearchWP calculated relevance using multiplier.
		add_filter( 'searchwp\query', function( $query, $args ) use ( $wpdb ) {
			// Calculate a CUSTOM relevance.
			$query['select'][] = "( SUM(relevance) * {$this->alias}mod ) AS {$this->alias}rel";

			// Implement a custom weight modifier based on date stored as meta value.
			$query['from']['select'][] = "
				(
					CASE
						WHEN UNIX_TIMESTAMP( {$this->alias}m.meta_value ) < UNIX_TIMESTAMP( NOW() )
							THEN {$this->modifier_past}
						WHEN UNIX_TIMESTAMP( {$this->alias}m.meta_value ) > UNIX_TIMESTAMP( NOW() )
							THEN {$this->modifier_future}
						ELSE 1
					END
				) AS {$this->alias}mod
			";

			// Custom JOINs.
			$query['from']['from'][] = "
				LEFT JOIN {$wpdb->posts} {$this->alias}p
					ON ( {$this->alias}p.ID = {$args['index_alias']}.id
						AND {$this->alias}p.post_type = '{$this->post_type}' )
			";
			$query['from']['from'][] = "
				LEFT JOIN {$wpdb->postmeta} {$this->alias}m
					ON ( {$this->alias}m.post_id = {$this->alias}p.ID
						AND {$this->alias}m.meta_key = '{$this->meta_key}' )
			";

			// Use our custom relevance to sort results by overriding the default ORDERBY.
			$query['order_by'] = [ "{$this->alias}rel DESC", ];

			return $query;
		}, 20, 2 );
	}
}

new My_SearchWP_Date_Modifier();