function _stepdate( &$date, &$timestamp, $step=array( 'day' => 1 )) {
    foreach( $step as $stepix => $stepvalue )
      $date[$stepix] += $stepvalue;
    $timestamp  = $this->_date2timestamp( $date );
    $date       = $this->_date_time_string( date('Y-m-d H:i:s', $timestamp ), 6);
    foreach( $date as $k => $v ) {
      if( ctype_digit( $v ))
        $date[$k] = (int) $v;
    }
  }