gistfile1.php

<?php

// hook_nodeapi

 if ($op == 'presave') {
    foreach ($node->field_reply as $key => $field) {

      if (strpos($field['value'], '<!-- time:') === FALSE) {
        $node->field_reply[$key]['value'] = $field['value'] . '<br /><!-- time: ' . date() . ' -->';
      }
    }
  }