update-wordpress-post-and-ACF-field.php

<?php
  require_once 'wp-load.php';
  $args = array(
      'post_type' => 'foto_archief',
      'posts_per_page' => -1
  );
  $items = get_posts( $args );
  if ($items ) {
    foreach( $items AS $item ) {
    $myData = get_field("date", $item->ID);


      update_field("date", $myData, $item->ID);

      echo "Updated post with ID ".$item->ID."<br/>";

    }
  }