facetwp date source converted to year

custom-hooks.php

<?php

/**
 * reindex after adding or updating this filter
 */
add_filter( 'facetwp_index_row', function( $params, $class ) {
	if ( 'date_as_year' == $params['facet_name'] ) { // change date_as_year to name of your facet
		$raw_value = $params['facet_value'];
		$params['facet_value'] = date( 'Y', strtotime( $raw_value ) );
		$params['facet_display_value'] = $params['facet_value'];
	}
	return $params;
}, 10, 2 );