<?php

// Add the following into the Custom Hooks plugin or your theme's functions.php
add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'date_range' == $params['facet_name'] ) {
        $params['facet_display_value'] = $params['facet_value'];
    }
    return $params;
}, 20, 2 );