Add bonus weight from Custom Field value in SearchWP

searchwp-customizations.php

<?php

// Add bonus weight from Custom Field value in SearchWP.
// @link https://searchwp.com/documentation/knowledge-base/add-relevance-weight-date/
add_filter( 'searchwp\query\mods', function( $mods ) {
	global $wpdb;

	// Custom Field name. Needs to store data as YYYYMMDD (ACF does this already).
	$my_meta_key  = 'date_field';
	
	$mod = new \SearchWP\Mod();
	$mod->set_local_table( $wpdb->postmeta );
	$mod->on( 'post_id', [ 'column' => 'id' ] );
	$mod->on( 'meta_key', [ 'value' => $my_meta_key ] );
	$mod->relevance( function( $runtime ) use ( $wpdb, $my_meta_key ) { return $wpdb->prepare( "
		COALESCE( ROUND( ( (
			UNIX_TIMESTAMP( {$runtime->get_local_table_alias()}.meta_value )
			- (
				SELECT UNIX_TIMESTAMP( meta_value )
				FROM {$wpdb->postmeta}
				WHERE meta_key = %s
				ORDER BY meta_value ASC
				LIMIT 1
			)
		) / 86400 ), 0 ), 0 )", $my_meta_key );
	} );

	$mods[] = $mod;

	return $mods;
} );