Because the function excel_format_due_date() is missing...

/**

• Gets the formatted due date.
• @param $p_due_date The date submitted
• @returns the due date
  */
  function excel_format_due_date( $p_due_date ) {
  return excel_prepare_string( date( config_get( 'short_date_format' ), $p_due_date ) );
  }