<?php

/**
 * reindex after adding or updating this filter
*/
add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'date' == $params['facet_name'] ) {
        $timestamp = strtotime( $params['facet_value'] );
        $formatted = date( 'Y-m-d', $timestamp );
        $params['facet_value'] = $formatted; // lower
        $params['facet_display_value'] = $formatted; // upper	
    }
    return $params;
}, 10, 2 );